Regex domains

Hi all!

I'm attempting to integrate this in a project that needs to allow origins that match a regex. From my reading of the README and some of the code, I don't _see_ this functionality, but I wanted to ask to make sure I wasn't overlooking it.

An example of when this functionality would be useful is for some front end deployments that we have to netlify that have unique hashes in the subdomain, such as `-my-slug.mydomain.com`. Ideally, I'd like to be able to set up rules that match any domain that matches that pattern.

If I'm not overlooking the functionality, is this something that you'd either consider supporting or would consider a PR for?