* feat(args): parse `countdown` by time
* fix lint
No `Default` for `AppTime` needed...
* app: pass `countdown_until` down
* fix `parse_duration_by_time` and `parse_duration`
to handle different formats they support
* fix(countdown): percentage panics
`Duration::ZERO` needs to be considered
* `DirectedDuration`
* fix comment
* rename arg: `countdown-target`
* `ss`->`mm`, fix formats, update README
* alias `--ct`