Allow formatting Cabal files

So this works, more or less. But there are some rough edges.

Since this uses STDIN rather than file-based input, the "expand" feature of cabal-fmt doesn't work. This means that comments like -- cabal-fmt: expand some-directory won't do what they're supposed to. This isn't a show stopper, but it's annoying.
- Even if cabal-fmt had a flag for this (like --file some/package.cabal), I wouldn't have any way of passing that to the command.
- Interestingly the Haskell formatter has a similar problem. Ormolu tries to infer language extensions based on the Cabal file. But since we're sending the file contents in through STDIN, it doesn't know which Cabal file is relevant. So we have to pass --no-cabal.
- This suggests that I need a way to send the file to the formatter.
This tries to format non *.cabal files that use the Cabal file format, like cabal.project. cabal-fmt has a flag for this (--no-cabal-file), but it's unclear how I could conditionally enable this flag. And cabal-fmt doesn't work very well in that scenario anyway.

tfausak / purple-yolk