Allow checking if formatting is okay

[mbhall88]

Have a parameter --check which does not do any formatting, but returns exit code 0 if no formatting is required and non-zero otherwise.

From black

  --check                         Don't write the files back, just return the
                                  status.  Return code 0 means nothing would
                                  change.  Return code 1 means some files
                                  would be reformatted.  Return code 123 means
                                  there was an internal error.