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
