[Feature] store pretrained data in XDG_CONFIG_HOME or /usr/share

Currently, `spleeter` (attempts to) download pretrained data to `pretrained_models` in the current directory. If `spleeter` is executed from different directories, the data is downloaded multiple times, wasting bandwidth and hard drive space.

Most programs are expected to store data in a subdirectory of `XDG_CACHE_HOME` or `/usr/share`.

I am aware of the `MODEL_PATH` environment variable that can be used as a workaround.