Only show recommendations for files changed in a PR

[ma-sadeghi]

Is there a way to force the wemake-python-styleguide action to show recommendation only for those files that have been changed in the PR? Currently, the action log shows recommendations for the entire repo.

[sobolevn]

I guess that it can be done via https://github.com/reviewdog/reviewdog#filter-mode
We can also support this as a parameter to our action.

Contributions are welcome!

Ping to @mgrachev