Tools like ALE and so forth generally pass the buffer of the editor
directly to the tool to edit and expect to read it back on stdout. I've
added the `-` to signal to add-trailing-comma to read data from stdin
and output to stdout.

I modeled this mostly off of black. Black doesn't have any error
handling around passing multiple files and `-`. Black also only briefly
mentions it in the docs.

I've also added a basic tests to check that a (mocked) stdin works.