Print a summary of the lint results

[Alexander]

When linting multiple files in CI it makes sense to only print the results of the failed lints. I suggested adding a flag for that in #17. One thing that concerns me is that if we add the flag there's no visibility in the amount of files that got linted in total (it might have linted none!). Let's print a summary at the end of the command. Something like:

Linted 300 files, found 0 errors