New rule: always-return

I've been chasing down some bugs on a project and I've encountered a number of cases where I have simply forgot to return a value. This is particularly bothersome with async code that uses Promises, which _reads_ like it works on side effects but actually requires the Promise to be returned.

This rule is not for everybody (hence off by default) but can be useful for those circumstances, or when one wants to study how much of their code is relying on side-effects.