Add custom program data paths

[MaarifaMaarifa]

This PR adds the ability to specify custom paths for cache, data and config directories of the program making it possible to store program data in locations that are not platform specific. The paths can be provided through the command-line or set via the config file as follows

[custom_paths]
data_dir = "/path/to/data"
cache_dir = "/path/to/cache"

Supplying paths via the command-line will override the ones in the config if available. Custom config directory is not configurable through the config file.