Clear the font cache on uninstall

[moi15moi]

When the user uninstall the program with pip uninstall FontCollector, it won't clear the font cache.
To actually clear the cache, the user needs to call FontLoader.discard_system_font_cache() and FontLoader.discard_generated_font_cache().

It seems that it isn't possible to run custom code when the user do a pip uninstall FontCollector. See Custom code on pip uninstall.
Because of that, I don't really know how I could clear the font cache on uninstall.