Allow git commit --fixup and --squash features

[mislav]

git commit flags --fixup and --squash are great in rebase-centric workflows, especially in combination with git config rebase.autosquash enabled. Unfortunately, commitplease component validation will abort these commits.

The solution would be to skip validation on commits that start with "fixup!" or "squash!". These are temporary commits that will get merged into others on the next rebase, so it makes no sense to perform validation on them at this moment.