feature request: indicate whether a file needs reformatting rather than reformatting it

A Debian Developer wants to incoporate stylish-haskell into a program called check-all-the-things which performs various automated tests on code. For this purpose, it would be useful if stylish-haskell had a flag, e.g. `--dry-run`, which caused it to print the names of files that would be changed during a normal run, without any files actually being changed. So you could do `stylish-haskell --dry-run src/*` or something like that.