Use $XDG_CACHE_HOME if defined

[Samueru-sama]

Fixes #354

[ulyszk]

Looks okay to me.

[Samueru-sama]

In the realm where files reside, A once-great spec now cast aside. Directories meant to guide with grace, Are scattered now, a disordered place.

Config, cache, and data too, No longer found where they once grew. Chaos reigns, the order lost, The XDG, forgotten, tossed.

Oh, what sorrow, what dismay, For those who followed the proper way. In this world, where norms decay, The XDG Base Dir Specification, by few, is heeded.