Add ability to include files by extension

[psanford]

When vendoring the packag github.com/gogo/protobuf, gvt currently removes the .proto source files when it fetches that package. I need these source files so that non-go languages can compile protobuf files that include gogoproto extensions.

I was thinking that there are other situations where you might want similar behavior. Maybe we could do this by using a whitelist of either file paths or file extensions in the dependency object in the manifest file.

If this sounds ok I'd be happy to send a pull request.