Add my old bash/zsh implementation of worldtime

This is fast -- noticeably faster than a Python implementation -- and requires
no exotic dependencies, though I could never get the fuzzy date/time parsing to
work to my liking.  Times would often need to be specified in full, complete
with a date, otherwise I'd end up with bizarre results.

Adding to the repo for the sake of hysterical reflection.  Perhaps someone else
will find it useful.

(Yes, the colour functions are missing.  Frankly, I couldn't be arsed digging
them out of my .zshrc library.)