Add `--check`

[bollwyvl]

Thanks for this useful tool!

Following the conventions of other CLI tools (e.g. isort, black) might make this a little more familiar and easy to integrate into larger static analysis suites.

I think a key one is --check, which would see if it would make a change, and maybe print out a diff, but definitely not update anything, and return a failing return code.

[gaborbernat]

PR welcome.

[bollwyvl]

#65 is up...