New option: send custom hostname via SMTP

[David Carlos Manuelda <stormbyte gmail com>]

I posted a wish bug in bugzilla
( https://bugzilla.gnome.org/show_bug.cgi?id=738155 ) with patches to
enable a new option that would allow user to specify if balsa should
send a custom hostname instead of real one when sending emails via SMTP.

I also think it is also a privacy issue and a good option to have.

Can someone review and test further those patches?

Thanks