Allow setting input file when reading from STDIN

[tfausak]

Fixes #51. Alternative to #65.

This adds a new --stdin-input-file FILE command-line option. It is intended to be used when reading a Cabal file from STDIN. The given FILE will be used to resolve relative references, which allows pragmas like -- cabal-fmt: expand some-directory to work even when reading from STDIN.

I borrowed the flag name from Ormolu. I'm not set on it — if there's a better name, let's change it.

Like #65, this does not let you set both --stdin-input-file and an actual input file. In other words, cabal-fmt --stdin-input-file x y is an error, even if x and y happen to be the same.

I think this is preferable to #65 because it should be easier to integrate with tools. It's very common to have the file path of the file you want to format, along with the file's contents in memory. Requiring a directory name would require calling something like dirname, which is just an extra unnecessary step.

[michaelpj]

Just to note that this is a problem for the HLS cabal-fmt plugin, which needs to call cabal-fmt with the contents of the file as they appear in the user's editor, not on disk. So we rely on the option to read from stdin.

[tfausak]

@phadej can you review this when you get a chance? Thanks!

[tfausak]

Is there anything that I can do to help move this along? This feature would allow me to replace hpack with cabal-fmt. It would also fix a problem with HLS.