CLI/show: mostly switch show to new config framework

CLI/show: mostly switch show to new config framework

This will need some cleanup when the transition completes, and we stop
passing notmuch_config_t structs to the subcommands.

Unlike the general case, we open the database in the subcommand, since
we don't know whether it should be opened read/write until we parse
the command line arguments.

Add a test to make sure passing config file on the command line is not
broken by these or future config related changes.