ADD settable zoneinfo directory functions

[wabscale]

In response to this issue https://github.com/HowardHinnant/date/issues/626 I've made only minor changes to the base library to allow for users to call a set_tz_dir function for setting a custom zoneinfo directory.

The use case for these changes are when you don't want to use the standard zoneinfo directory at /usr/share/zoneinfo. Users can call the set_tz_dir function before initializing the database. If the path the user has specified does not exist or is not a directory, it falls back to the standard paths the library looks for.