Build step for merging json schema files

[ashfame]

Added a custom webpack plugin EmitMergedJsonPlugin that:

• Reads all JSON files from a specified source directory
• Merges them into a single JSON object
• Outputs the merged schema to multiple locations:
  • Build directory
  • WordPress plugin src dir

Next, we need to figure out the output paths, but that can happen in the PR which starts using the generated schema file.

closes #144

---

Output sample:

cat build/firefox/schema.json
{
  "page": {
    "title": "Page",
    "fields": {
      "title": "String",
      "content": "String"
    }
  },
  "post": {
    "title": "Blog Post",
    "fields": {
      "date": {
        "type": "Date",
        "description": "date of the post",
        "required": true
      },
      "author": "String",
      "title": "String",
      "content": "String"
    }
  },
  "product": {
    "title": "Product",
    "fields": {
      "title": "ProductString",
      "description": "String",
      "price": "currency"
    }
  }
}

[psrpinto]

Nice, thanks for working on this!

[psrpinto]

Looks great!