`scriv collect` should check category names

[SpecLad]

Currently it seems that scriv doesn't care if the headers in the changelog fragments match the configured category names. So if I make a typo and write e.g. "### Secruity", scriv will just assume that it's a category of its own.

It would be useful if scriv were to error out when encountering such fragments.

[kurtmckee]

Would it be helpful for such a check to be a pre-commit hook, so that developers can catch this at PR submission, rather than when they're trying to cut a release?

[SpecLad]

I think it should be part of scriv collect, but this subcommand could have a --dry-run/--check option, which would then be usable as a pre-commit hook.