notmuch reply: Add (incomplete) reply command

Reviewed-by: Carl Worth <cworth@cworth.org>
Keith wrote all the code here against notmuch before notmuch.c was
split up into multiple files. So I've pushed the code around in
various ways to match the new code structure, but have generally tried
to avoid making any changes to the behavior of the code.

I did fix one bug---a missing call to g_mime_stream_file_set_owner in
show_part which would cause "notmuch show" to go off into the weeds
when trying to show multiple messages, (since the first stream would
fclose stdout).