Excluding files from dependency

[ebroder]

I have a dependency which includes a handful of optional CSS files, all of which are included in the "main" section of its bower.json. Several of them are fairly large, and I'd like to be able to exclude them from the resulting Meteor artifact.

Happy to try and sketch out a patch here, but would appreciate suggestions on what the interface should look like. My strawman would be something like

  exclude: {
    'dependencyName': [
      'build/js/file1.js',
    ]
  }