Add preBundleCb option

Type: `Function (b)`

An optional callback function, that will be called before bundle completion.
`b` is the `browerify` instance that will output the bundle.

Example:

```
options: {
  preBundleCB: function (b) {
    b.on('file', function(file) {
      grunt.log.ok(file);
    });
  }
}
```