Display extra headers for emacs-mua 2 new patches

[Johan Parin]

This is a refinement of my first two submissions, cleaned up and also
working with reply, which the first versions were not:

1) notmuch-show returns all headers
2) notmuch-show returns extra headers configured by a command line
option, which is passed from the emacs mua.

As for the second, I changed the command line option to specify extra
headers instead of the full set of headers. This is a lot easier,
since some headers have special treatment in format_headers_sprinter,
and it is also called from reply.

This patch is very similar to the one storing the option in the
database and using a global database pointer, except that elisp is
added to pass the command line flag.

There is still one global used in notmuch-show.c in order to pass down
the command line argument. This really is necessary since changing the
signature of format_headers_sprinter is not really feasible as shown
by the patch which passed around the database pointer.

I hope that having this global is more acceptable than having a
global database pointer.

So in summary, the patch variants I have submitted:

a) notmuch-show.c returns all headers in message.
   Con: There are concerns about potential performance impact
b) notmuch-show.c returns extra headers as configured in database.
   Con: Uses a file global database pointer in notmuch-show.c
c) notmuch-show.c returns extra headers as specified by a command line
   flag. This flag is set by the emacs-mua based on
   notmuch-message-headers if that differs from the default list.

I don't see any con with c) really.

I have only concerned myself with sexp output since I personally use
the emacs-mua. But I think modification for the other formats can be
done as a later step.

/Johan