notmuch-show: Add content-description output pair

notmuch-show: Add content-description output pair

Parse and display the Content-Description header [1,2] so UIs
rendering message parts have easy access to description strings.
Extracting the value is a two-step process (extract [3] and decode
[4]).

[1]: http://tools.ietf.org/html/rfc2045#section-8
[2]: http://tools.ietf.org/html/rfc2183#section-3
[3]: https://developer.gnome.org/gmime/stable/GMimeObject.html#g-mime-object-get-header
[4]: https://developer.gnome.org/gmime/stable/gmime-gmime-utils.html#g-mime-utils-header-decode-text