Add summary info

[okainov]

I think it would be useful to have some summary of the execution in addition to exitcode, like

XXX files checked
Y of them are not formatted correctly / Everything is OK

With maybe even list of files having issues.

The use case is when I integrate the script i.e. in git pre-commit hook and I don't want to see the full diff, but I'd like to have some useful human-readable output, so developers know where are the issues

[Sarcasm]

For a git pre-commit hook, I think it would be more suitable to use:

git clang-format --diff

[Sarcasm]

But still, summary info isn't a bad idea.