[Feature proposal] ActionMailbox configuration for InboundEmail class

I’ve found that I keep extending ActionMailbox::InboundEmail class to support my cases (additional relationships, etc.) and I find this rather wrong.

I propose that InboundEmail class should be configurable, defaulting to ActionMailbox::InboundEmail, obviously. This way I (and others with needs similar to mine) can either subclass it or even have their own class (this is especially easy since most of the functionality of ActionMailbox::InboundEmail comes from included modules).

I am happy to prepare a PR to implement this.

Thanks for the information.