This commit introduces the notmuch backend.
The backend is conditionally compiled in if the "notmuch" tag is provided.
Most of the message types are implemented, with the notable exceptions
of DeleteMessages as well as any copy / move / append type.
Reason being, that those aren't normally applicable in a notmuch based workflow.
Changes v2 --> v3, based on review comments
* Use account config for configuration
Reto Brunner