Feature: clock offset, and legend/title

[poetaman]

Add a way to pass a clock offset, and a legend/title. That will enable using rsclock as a world-clock & users will be able to see the times from different regions of the world in split terminal screens like supported by tmux.

Currently only one location's time is visible, with ability to specify clock offset, each of the clocks in image below would be a different city:

[Kyllingene]

Properly handling timezones is an absolute pain. It's not just an offset: its daylight savings time, it's leap years, leap days, and leap seconds, it's countries changing timezones, DST, etc. Chrono handles timezones for you, so it might be possible to enter a supported timezone.

[poetaman]

@Kyllingene Timezones are also supported in https://github.com/poetaman/arttime.

[valebes]

Probably it could be easy to implement using Chrono-Tz crate.