pretty.c: rework describe options parsing for better extensibility

pretty.c: rework describe options parsing for better extensibility

It contains option arguments only, not options. We would like to add
option support here too, but to do that we need to distinguish between
different types of options.

Lay out the groundwork for distinguishing between bools, strings, etc.
and move the central logic (validating values and pushing new arguments
to *args) into the successful match, because that will be fairly
conditional on what type of argument is being parsed.

Signed-off-by: Eli Schwartz <eschwartz@archlinux.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>