A fairly sprawling commit:
* Move config to its own crate to facilitate making things more modular
* Crates/modules can now provide a registration function that can be
used to setup functions in the lua environment
* Message crate now has a dkim module with a signer type that can
be loaded from lua; signers are cached (with ttl) and shareable
* Message now has a dkim_sign method that does the signing, as well
as methods for appending and prepending headers that are necessary
to support signing.