Allow including files

[tfausak]

More or less blocked by #66.

It would be great to support explicitly including files along with excluding them. I think this would support two main use cases: separating files by name, like "internal", and separating files by extension, like "hsig". For example:

-- Separating internal modules.

-- cabal-gild: discover --exclude=**/Internal.hs
exposed-modules: ...
-- cabal-gild: discover --include=**/Internal.hs
other-modules: ...

-- Separating signatures.

-- cabal-gild: discover --exclude=**/*.hsig
exposed-modules: ...
-- cabal-gild: discover --include=**/*.hsig
signatures: ...

It should be possible to use both flags at the same time. First things are included, then they're excluded. The default behavior is like --include=**/*.