Support for index manifests

Hello! I've found that this awesome package doesn't have support for index manifests, which are commonly used in complicated Rails apps:

http://guides.rubyonrails.org/asset_pipeline.html#using-index-files

So, I've added it here. Changes are pretty straightforward, but if you want to implement it in better way, I would highly appreciate it.

Thanks, Sergey.

ps. I've also have added check for matching depTree files to specified extensions (maybe we also need specs for this).