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

Usage example:

```javascript
var browserifyFilter = require('broccoli-browserify');
return browserifyFilter(tree, {
  entries: [
    require.resolve(require('traceur').RUNTIME_PATH),
    './main-module'
  ],
  bundle: { debug: true },
  require: [
    ['./main-module', { expose: 'main-module' }],
    ['jquery-browserify', { expose: 'jquery' }]
  ],
  outputFile: 'main-module.js'
});
```