fix(clock): show locale-formatted time

[rchl]

This makes clock directive use time formatting from the currently loaded locale. Also refactored it to be a bit cleaner.

Also fallback to locale date for HEADER.DATE/DATETIME. But since the example config has formatting explicitly defined, this will probably have no effect on existing configurations.

[rchl]

BTW. When merging, I prefer to use "Squash and merge" as that doesn't introduce unnecessary merge commits and cleans up the branch.

[alphasixtyfive]

Thank you!