Read from stdin

[eeue56]

Outfoxes #66

I also changed the spec slightly -

• If elm-format - run, then read from stdin
• If elm-format --stdin run, then read from stdin
• If --output is set, then save to that output file

Ideally, I think I'd like it so that there was only a --stdin since it's possible to accidentally do - and be trapped in a stdin reader. I added the --stdin option, so that it self documents the ability to automatically it print it out when the file is run with no args or --help is set.

The only thing I haven't done is update the docs as part of this PR - I wanted to check with @avh4 whether we want to keep elm-format - as a thing or to ditch it first.

[avh4]

Okay, I'm sold on removing elm-format - in favor of --stdin. I think that fits well with Elm's philosophy of preferring well-documented interfaces.

Also, there is currently behavior where if multiple files are specified, then it is an error to give --output. We should handle this similarly: if --stdin is given, then it should be an error to provide any other filenames (and show an appropriate message).

Are there any other edge-cases we need to consider?

[eeue56]

Okay, done and done - this makes the code a lot cleaner, so I'm happy with this solution. As it stands, I don't think there's any other edge cases. There might be when more support for tooling is worked on, because of how the tooling wants to work, but I can't think of any right now.

[avh4]

Looks great, thank you!