Allow setting custom proj_data dirs

[kylebarron]

We expect to soon have working wheels (mac and linux only) of proj that include a minimal proj_data collection (the same as pyproj). We should also allow users to set a custom proj_data directory so that if they need datum grids from https://github.com/OSGeo/PROJ-data they can have them downloaded instead of using a remote CDN.

This amounts to copying most of the pyproj file here: https://github.com/pyproj4/pyproj/blob/329f2017e303a577ca1d4f3e116d9131c3666ce2/pyproj/datadir.py