Add Morpher::Hybrid

@mbj if they don't implement `#inverse` then bidirectional mappers like https://github.com/snusnu/mom/blob/master/lib/mom/mapper.rb#L31-L34 can't use hybrid morphers. I have to say tho, that I found no recent usecase for such a bidirectional mapper, that's why I've already made it so that `mom/mapper` doesn't get required by default.