Change the default cache directory

[MosGeo]

Currently jgo uses ~/.jgo as the cache directory. It would be great if this folder location can be modified by the user through Python or through environmental variable.

Rational

By letting the user define the folder location, portablity can be improved. For example, the cache directory can be moved easily with with the project to a new computer and it would just work without redownloading the required files or manually copying the .jgo folder to the user directory.

Workaround

Write some code in python at the start of the application to copy .jgo folder if it is not found in the user directory. It is not ideal as it might conflict with exsiting folder. Also, it feels like a hack.

[jayvdb]

I believe it can be configured using env var JGO_CACHE_DIR