Add feature to use system tzdata instead of vendoring it

[triallax]

Linux distros already ship tzdata, it would be nice if Numbat could optionally be built to use it instead of vendoring tzdata. This would also reduce the binary size quite a bit (not that it was ever that big anyway).

chrono-tz issue: chronotope/chrono-tz#73

[sharkdp]

Looks like https://github.com/kennytm/tzfile already implements everything as required. We could use that on unix. And use chrono_tz on other platforms.