Permit `--fixup` and `--squash` commits

[danbernier]

commitlint currently fails the messages auto-generated by git for --fixup and --squash commits.

I think commitlint should always permit these commit messages.

Since the point of these commits is to be rebased into other commits, their messages won't be in the commit history for terribly long. And they're already an established part of the git ecosystem.

I'm currently working around this by using -n every time I --fixup, but it's a friction point each time I forget.