Add files to watch-list in karma preprocessor

See cross-post here: http://stackoverflow.com/questions/37795680

I expect this is a documentation issue.
### Expected behavior

One should be able to add files to the list of watched files from a Karma preprocessor.
### Actual behavior

There is no clear indication of how to accomplish this, and the expected mechanisms fail, namely:
1. adding to `config.files` does not lead to watching
2. including `fileList` on the `$inject` for the preprocessor plugin fails with a circular dependency
### Enviroment Details
- Karma version (output of `karma --version`): 0.13.22
### Steps to reproduce the behaviour

See [linked Stackoverflow question](http://stackoverflow.com/questions/37795680)