export categories with the npm module

[zeke]

...so the website doesn't have to deal with it.

Example:

[
   {
      "count": 4,
      "name": "Books",
      "slug": "books"
    },
    {
      "count": 7,
      "name": "Business",
      "slug": "business"
    },
    {
      "count": 72,
      "name": "Developer Tools",
      "slug": "developer-tools"
    }
]

We can use the already-depended-on slugg to generate the slugs.

To avoid making this a breaking change, we can just keep the default behavior for require('electron-apps') and access the categories using require('electron-apps/categories')

cc @jkleinsc