Add `browserify.transform` field to the package.json

So that `quill` can be browserified from the command-line after a simple `npm install quilljs` command.

See: http://stackoverflow.com/a/16113666/376773

So now a user can `npm install quilljs`, and then given a file `t.js` do something like this:

``` js
var Quill = require('quilljs');

var editor = new Quill('#editor');
editor.addModule('toolbar', { container: '#toolbar' });
```

And then ultimately compile it via browserify like this:

``` bash
$ browserify --extension=".coffee" t.js > bundle.js
```