WIP: feed: Add Content-Description to digest parts

WIP: feed: Add Content-Description to digest parts

Copy the message Subject to the MIMEMessage's Content-Description
[1,2].  We already set the MIME types for the attachements (with
MIMEMessage).  Mutt [3] seems to look inside attached message parts
and extract their subject as a description, so the current xkcd feed
looked like:

  A     1 Questions for God               [message/rfc822, 7bit, 0.6K]
  A     2 Inexplicable                    [message/rfc822, 7bit, 0.6K]
  A     3 Theft                           [message/rfc822, 7bit, 0.7K]
  A     4 Actually                        [message/rfc822, 7bit, 0.7K]

However, in the notmuch-show Emacs mode [4] it looked like:

  xkcd.com: <author> <user@rss2email.invalid> (20 mins. ago) (inbox)
  Subject: digest for xkcd
  To: wking@tremily.us
  Date: Sat, 18 Jan 2014 17:18:20 +0000

  [ multipart/digest ]
  [ message/rfc822 (hidden) ]
  [ message/rfc822 (hidden) ]
  [ message/rfc822 (hidden) ]
  [ message/rfc822 (hidden) ]

which is not very informative.  With this commit, the Mutt rendering
is unchanged, and the notmuch-show-mode rendering is (WIP:
unchanged?):

  xkcd.com: <author> <user@rss2email.invalid> (Today 08:01) (inbox)
  Subject: digest for xkcd
  To: wking@tremily.us
  Date: Sun, 02 Feb 2014 16:01:32 +0000

  [ multipart/digest ]
  [ message/rfc822 (hidden) ]
  [ message/rfc822 (hidden) ]
  [ message/rfc822 (hidden) ]
  [ message/rfc822 (hidden) ]

I'll ping the Notmuch list about this...

[1]: http://tools.ietf.org/html/rfc2045#section-8
[2]: http://tools.ietf.org/html/rfc2183#section-3
[3]: http://www.mutt.org/
[4]: http://notmuchmail.org/emacstips/

Reported-by: Victor J. Orlikowski <vjo@duke.edu>
Signed-off-by: W. Trevor King <wking@tremily.us>