notmuch/email: allow extra headers to be shown.

notmuch/email: allow extra headers to be shown.

When viewing an email, it can be useful to be some extra headers without
having to look through the complete raw headers.

There is also information that notmuch knows which doesn't exist in the
headers.

So add an "extras" button which causes both doc-email and notmuch to add
extra headers that might be interesting.
 Message-ID
 In-Reply-To
 References
 Filename
 Thread-ID

Signed-off-by: NeilBrown <neil@brown.name>