allow to change the cache location

[omar-polo]

On unix we can change the cache path by setting XDG_CACHE_HOME before running plakar. ~On windows, I don't think it's possible.~ edit, it is possible on windows too, albeit it's more complicated: (powershell example)

$env:LocalAppData = "E:\path\to\cache"
plakar.exe ...

In any case, it's a bit cumbersome, so I think we should allow changing the cache path via a configuration knob.