Allow patterns to use patterns

[Swahvay]

I have a use case where I have a very low-level pattern to be used with multiple other patterns, but the API doesn't support it. There would only be a few fields in this low-level pattern, so not the end of the world if it's not something you want to support. I know you'd then have to do things like cyclical inheritance checking or something which wouldn't be fun.

[lolopinto]

related: https://github.com/lolopinto/ent/issues/585