Implement new library to retrieve static assets URL

.eslintrc.json

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+{
+  "extends": "airbnb",
+  "env": {
+    "jest": true
+  }
+}

.gitignore

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+node_modules
+coverage
+lib

.travis.yml

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+language: node_js
+node_js:
+  - "8"
+notifications:
+  email: false

README.md

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+# buffer-js-static-assets
+
+[![Build Status](https://travis-ci.org/bufferapp/buffer-js-static-assets.svg?branch=master)](https://travis-ci.org/bufferapp/buffer-js-static-assets)
+
+## Installation
+
+Install the package
+
+```
+npm install @bufferapp/static-assets
+```
+
+## Usage
+
+To utilize the `staticUrl` function, you'll need to first initialize the
+static assets manager:
+
+```js
+const { initStaticAssets } = require('@bufferapp/static-assets')
+
+initStaticAssets()
+  .then(() => {
+    // Now that we have loaded the static assets we can serve requests
+    app.listen(8080, () => console.log('Server started!'))
+  })
+```
+
+Alternatively, you can call the `initStaticAssetsSync` function to perform
+a synchronous load of the static assets file which is blocking:
+
+```js
+initStaticAssetsSync()
+app.listen(8080, () => console.log('Server started!'))
+```
+
+Once the library is initialized, using the exported `staticUrl` function:
+
+```js
+const { staticUrl } = require('@bufferapp/static-assets')
+
+app.get('/sweet', (req, res) => {
+  res.render('sweet-template', {
+    headerImage: staticUrl('img/sweet-header4.png'),
+    script: staticUrl('js/sweet-bundle.js'),
+  })
+})
+```
+
+### Options
+
+We have a few configuration options that can be passed to `initStaticAssets` in
+a single object argument (ex. `initStaticAssets({ path: '/static' })`):
+
+* `staticAssetVersionsFilename` - The name of the static asset manifest file.
+* `staticAssetDirLocal` - The relative path where static assets are served during local dev. Usually via express-static.
+* `staticAssetDirCDN` - This is the directory relative to your working directory where your static assets are saved. This is used to look up static files in the manifest (See manifest section below).
+
+### Static Asset Manifest
+
+The format of the static asset manifest should be the same as default JSON
+format that [`buffer-static-upload`](https://github.com/bufferapp/buffer-static-upload)
+outputs.
+
+This format is a key-value pair of:
+
+```json
+"relative path to local file from working directory": "remote URL for that file"
+```
+
+Here's an example:
+
+```json
+{
+  "public/css/style.css": "https://static.buffer.com/my-app/public/css/style.1234567890.css",
+  "public/img/brand.png": "https://static.buffer.com/my-app/public/img/brand.png"
+}
+```
+
+## License
+
+MIT