Create a default feeds importer for mailhandler

A default feeds importer for mailhandler should be provided as a separate module, something like "mailhandler_default" which defines a default feeds importer which uses the mailhandler fetcher, parser, and node processor, and which has a basic mapping set up in the node processor mappings settings. The goal is to provide users with a default feeds importer which is setup properly and which they can customize to a minimum degree and start using with one of their mailboxes.