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: https://github.com/chronotope/chrono-tz/issues/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.

[sharkdp]

Decided I'm not going to spend time working on this.

[sharkdp]

With #511, this is now supported, actually.