Added functionality for b.require(<file>)

[b.require()](https://github.com/substack/node-browserify#brequirefile-opts) exposes a package so that it can be required outside of the bundle. This is useful when you need to stub/mock dependencies during testing.

I have built this into `grunt-browserify` so that it can be used like so:

```js
browserify: {
  testable: {
    src: ['lib/index.js'],
    dest: 'build/build-test.js',
    options: {
      require: [
        'event',
        './lib/*/*.js'
      ]
    }
  }
}
```

Enabling:

```js
window.require('event');
window.require('./lib/dir/file1.js');
window.require('./lib/dir/file2.js');
```