RFC: User-Agent header

[Sebastian Spaeth]

notmuch is (mostly) not responsible for sending email. However, people
using the emacs frontend use notmuch to create the reply.

Am I the only one who is sometimes curious as to what mail agents others
use? Would it be useful to insert a header to notmuch reply analog to:

User-Agent: Mutt/1.5.13 (2006-08-11)

We could reuse the same version string that we use for the release (or
the git string that was used to build notmuch). I can use this to create
nice stats :).

No patch yet, just asking if this is a good idea or not.

Sebastian