Add `transform` option to call browserify.transform()

Notes:
* The implementation of this 'transform' option is identical in style to the recent 'require' option commit: https://github.com/gingerhendrix/broccoli-browserify/pull/4
* I also fixed a bug with the 'require' option, which should use an empty [] as default, not {}

Example transform config, using [coffeeify] (https://github.com/jnordberg/coffeeify) and [aliasify](https://github.com/benbria/aliasify) transforms:

```
var appJs = browserify('app', {
    entries: [ './app.coffee' ],
    outputFile: 'app.js',
    browserify: {
        extensions: [ '.coffee' ],
    },
    transform: [ [ 'coffeeify' ], [ { global: true }, 'aliasify' ] ]
});
```