[notmuch] Prototype of --show option to control what is shown.

[david at tethera.net]

Here is a set of 3 patches to implement (some) output control.  

[PATCH 1/3] rename option to select output format to --format from --output.
[PATCH 2/3] notmuch-show.c: make calls to format functions conditional

The first two are essentially suggestions for Scott.  In particular the first 
is not needed at all for this little experiment.

[PATCH 3/3] notmuch-show.c: prototype tabular output format, with output control

The third implements a new "tabular" output format, and shows how it
might be conditionally controlled.  At the moment the only
possibilities to output are message-id, match flag, and filename.
This is mainly a matter of laziness.  I didn't want to go too crazy
before Scott's reached some final(ish) version.