command-line-arguments.[ch]: new argument parsing framework for notmuch.

As we noticed when Jani kindly converted things to getopt_long, much
of the work in argument parsing in notmuch is due to the the key-value
style arguments like --format=(raw|json|text).

The framework here provides positional arguments, simple switches,
and --key=value style arguments that can take a value being an integer,
a string, or one of a set of keywords.