Add possibility to use tzdata provided by OS

[liveforeverx]

Making it more operations compliant, and allow to update all tzdata in one run.
Add possibility to read tzdata from OS (which is on every Linux already installed and updated).

Make it sense?

[lau]

Usually OS distributions are relatively slow to provide updates to the timezone data. And in practise many systems get way behind on updates. I would run a server in UTC anyway so I would not use the tzdata of the OS on a server.

Do you have an application where you use the OS timezone for anything?

Currently in with Elixir Tzdata you can switch automated updating on and off in config. And you can define a directory where the data is stored (the .ets files in the priv dir). If you want to you can have automated updates off. And then it should be possible to trigger the updates manually. A delayed and more manual or semi-automatic triggering of updates in Tzdata could be explored.