Allow full class names (with namespaces) for Action Mailbox routes

Currently, when declaring mailbox routes, it’s only possible to pass a string/symbol, which gets camelized into a class name:

`routing /^save@/i => :forwards
routing /@replies\./i => :replies```


Would it make sens to allow the following syntax:

routing /noa/i, class_name: ‘LongHaul::NOAMailbox’
routing /delivery/i, class_name: ‘LastMile::DeliveryMailbox’


This way, all the mailboxes wouldn’t have to live in the same directory.