Add new date command

[vinc]

Back when MOROS was using cargo-xbuild we couldn't compile the time crate (time-rs/time#268) but now that it doesn't use it anymore (#83) we can compile it.

So we can now have a new date command that accepts formatting (https://time-rs.github.io/time/time/index.html#formatting).

> date
2020-11-08T23:47:02

> date "%A, %B %d, %Y"
Sunday, November 08, 2020

[vinc]

With nanoseconds precision:

> date "%F %H:%M:%S.%N"
2020-11-09 00:20:17.307953119

[vinc]

Added an env command to set a TZ env var used by the date command to display the date with a UTC offset.

In the future we'll want to import a time zone database to find the UTC offset for a given date and time zone, we could also add a geosearch to the database to use latitude and longitude instead of strings like Europe/Paris and be compatible with the geotime command that use longitude.

At the moment MOROS will continue to be generally unaware of time zones and assume the RTC is using locale time.