Support `--diff` option

[HideakiImamura]

Thanks for the great project! Recently, Optuna, which is a hyperparameter optimization framework, has started to use the blackdoc in CI.

The black supports the --diff option to show the difference before and after formatting. This option is enabled in the CI of Optuna. However, blackdoc does not. This option is useful for PR authors to find out why CI has failed. I would love to have blackdoc support this option.

[keewis]

this has been on my roadmap for quite some time now (see points 4 and 5 in #33 (comment)).

Note to self: use difflib.unified_diff