[Feature] Forks filtering

[t0mmili]

1. New env var GHS_FILTER_FORKS.
2. Tests for new functionality.

[vladkens]

Hi, @t0mmili. Thanks for the PR. This is a useful functionality. The only thing I'm not sure about is a separate environment for configuration (because there's also archive and private flags). I was also thinking about additional rules when working on filtering, but haven't figured out the syntax yet. What do you think about this format:

GHS_FILTER=vladkens/*,!fork,!private,!archive

If it looks okay I can update your PR with this rules.

[t0mmili]

@vladkens, that was actually my initial idea to use !fork 😆 Personally it felt odd and confusing at first, from the user perspective. But on the other hand, maybe it makes more sense to keep filtering in one var. Go ahead and update the PR. Thanks!