Add new plugin to handle excel files (#2343)

* Add new plugin to handle excel files

* Run format

examples/using-excel/.eslintrc

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+{
+  "env": {
+    "browser": true
+  },
+  "globals": {
+    "graphql": false
+  }
+}

examples/using-excel/.gitignore

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+public
+.cache
+node_modules

examples/using-excel/README.md

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+# using-excel
+
+https://using-excel.gatsbyjs.org

examples/using-excel/gatsby-config.js

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+module.exports = {
+  siteMetadata: {
+    title: `gatsby-example-using-excel`,
+    description: `Blazing-fast React.js static site generator`,
+  },
+  plugins: [
+    `gatsby-transformer-excel`,
+    {
+      resolve: `gatsby-source-filesystem`,
+      options: {
+        path: `${__dirname}/src/data`,
+        name: `data`,
+      },
+    },
+  ],
+}

examples/using-excel/package.json

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+{
+  "name": "using-excel",
+  "private": true,
+  "description": "Gatsby example site using using-excel",
+  "author": "SheetJS <dev@sheetjs.com>",
+  "dependencies": {
+    "gatsby": "latest",
+    "gatsby-link": "latest",
+    "gatsby-source-filesystem": "latest",
+    "gatsby-transformer-excel": "latest"
+  },
+  "license": "MIT",
+  "main": "n/a",
+  "scripts": {
+    "develop": "gatsby develop",
+    "build": "gatsby build"
+  }
+}

examples/using-excel/src/pages/index.js

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+import React from "react"
+
+class IndexComponent extends React.Component {
+  render() {
+    const data1 = this.props.data.allLettersXlsxSheet1.edges
+    const data2 = this.props.data.allLettersXlsxSheet2.edges
+    return (
+      <div>
+        <table>
+          <thead>
+            <tr>
+              <th colSpan="2">Sheet1</th>
+            </tr>
+            <tr>
+              <th>Letter</th>
+              <th>ASCII Value</th>
+            </tr>
+          </thead>
+          <tbody>
+            {data1.map((row, i) => (
+              <tr key={`${row.node.value} ${i}`}>
+                <td>{row.node.letter}</td>
+                <td>{row.node.value}</td>
+              </tr>
+            ))}
+          </tbody>
+        </table>
+        <table>
+          <thead>
+            <tr>
+              <th colSpan="2">Sheet2</th>
+            </tr>
+            <tr>
+              <th>Letter</th>
+              <th>ASCII Value</th>
+            </tr>
+          </thead>
+          <tbody>
+            {data2.map((row, i) => (
+              <tr key={`${row.node.value} ${i}`}>
+                <td>{row.node.letter}</td>
+                <td>{row.node.value}</td>
+              </tr>
+            ))}
+          </tbody>
+        </table>
+      </div>
+    )
+  }
+}
+
+export default IndexComponent
+
+export const IndexQuery = graphql`
+  query IndexQuery {
+    allLettersXlsxSheet1 {
+      edges {
+        node {
+          letter
+          value
+        }
+      }
+    }
+    allLettersXlsxSheet2 {
+      edges {
+        node {
+          letter
+          value
+        }
+      }
+    }
+  }
+`

packages/gatsby-transformer-excel/.gitignore

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+/*.js
+!index.js
+yarn.lock

packages/gatsby-transformer-excel/.npmignore

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+# Logs
+logs
+*.log
+
+# Runtime data
+pids
+*.pid
+*.seed
+
+# Directory for instrumented libs generated by jscoverage/JSCover
+lib-cov
+
+# Coverage directory used by tools like istanbul
+coverage
+
+# Grunt intermediate storage (http://gruntjs.com/creating-plugins#storing-task-files)
+.grunt
+
+# node-waf configuration
+.lock-wscript
+
+# Compiled binary addons (http://nodejs.org/api/addons.html)
+build/Release
+
+# Dependency directory
+# https://www.npmjs.org/doc/misc/npm-faq.html#should-i-check-my-node_modules-folder-into-git
+node_modules
+*.un~
+yarn.lock
+src
+flow-typed
+coverage
+decls
+examples

packages/gatsby-transformer-excel/README.md

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+# gatsby-transformer-excel
+
+Parses Excel files into JSON arrays.
+
+## Install
+
+`npm install --save gatsby-transformer-excel`
+
+## How to use
+
+```javascript
+// In your gatsby-config.js
+plugins: [
+  `gatsby-transformer-excel`,
+]
+```
+
+## Parsing algorithm
+
+The parsing is powered by the [SheetJS / js-xlsx](https://git.io/xlsx) library.
+Each row of each worksheet is converted into a node whose keys are determined
+by the first row and whose type is determined by the name of the worksheet.
+
+So if your project has a `letters.xlsx` with two worksheets:
+
+```
+------ Sheet1 ------
+/|    A    |   B   |
+-+---------+-------+
+1| letter  | value |
+-+---------+-------+
+2|    a    |   97  |
+-+---------+-------+
+3|    b    |   98  |
+
+------ Sheet2 ------
+/|    A    |   B   |
+-+---------+-------+
+1| letter  | value |
+-+---------+-------+
+2|    A    |   65  |
+-+---------+-------+
+3|    B    |   66  |
+```
+
+the following nodes would be created:
+
+```javascript
+[
+  { letter: 'a', value: 97, type: 'LettersXlsxSheet1' },
+  { letter: 'b', value: 98, type: 'LettersXlsxSheet1' },
+  { letter: 'A', value: 65, type: 'LettersXlsxSheet2' },
+  { letter: 'B', value: 66, type: 'LettersXlsxSheet2' },
+]
+```
+
+## How to query
+
+You'd be able to query your letters like:
+
+```graphql
+{
+  allLettersXlsxSheet1 {
+    edges {
+      node {
+        letter
+        value
+      }
+    }
+  }
+}
+```
+
+Which would return:
+
+```javascript
+{
+  allLettersXlsxSheet1: {
+    edges: [
+      {
+        node: {
+          letter: 'a'
+          value: 97
+        }
+      },
+      {
+        node: {
+          letter: 'b'
+          value: 98
+        }
+      },
+    ]
+  }
+}
+```
+

packages/gatsby-transformer-excel/index.js

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+// noop

packages/gatsby-transformer-excel/package.json

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+{
+  "name": "gatsby-transformer-excel",
+  "version": "0.1.0",
+  "description": "Gatsby transformer plugin for Excel spreadsheets",
+  "main": "index.js",
+  "scripts": {
+    "build": "babel src --out-dir . --ignore __tests__",
+    "watch": "babel -w src --out-dir . --ignore __tests__",
+    "prepublish": "cross-env NODE_ENV=production npm run build"
+  },
+  "keywords": [
+    "gatsby",
+    "excel",
+    "gatsby-plugin"
+  ],
+  "author": "SheetJS <dev@sheetjs.com>",
+  "license": "MIT",
+  "dependencies": {
+    "babel-runtime": "^6.26.0",
+    "xlsx": "^0.11.5"
+  },
+  "devDependencies": {
+    "babel-cli": "^6.26.0",
+    "cross-env": "^5.0.5"
+  }
+}