wip: add a `--tz` option to the `avail` command

[flavorjones]

Related to #191

This adds a --tz option to allow the output format to be in one (or more) specific timezones, rather than the calendar timezones.

Things that still need work:

• [ ] the option should either only be available on commands that respect it, or all commands need to respect it
• [ ] need to add some tests