Before, the information needed to display different parts of the UI was
tightly coupled to the specific messages being sent back and forth to
the backend worker. Separating out a models package allows us to be more
specific about exactly what a backend is able to and required to provide
for the UI.

Credit for this fix goes to Reto; I guess if we're not gonna be mutt
we should probabaly do things correctly.

The unsubscribe command, available when in a message viewer context,
enables users to easily unsubscribe from mailing lists.

When the command is executed, aerc looks for a List-Unsubscribe header
as defined in RFC 2369. If found, aerc will attempt to present the user
with a suitable interface for completing the request. Currently, mailto
and http(s) URLs are supported. In the case of a HTTP(S) URL, aerc will
open the link in a browser. For mailto links, a new composer tab will be
opened with a message filled out according to the URL. The message is
not sent automatically in order to provide the user a chance to review
it first.

Closes #101

Tab completion currently only works on commands. Contextual completion
will be added in the future.