Use cache_dir ($XDG_CACHE_HOME, etc) as location to store the temporary swp file when saving

[elkowar]

Currently we're creating a .swp file in the files directory temporarily whilst saving. This should probably happen somewhere else, in a standardized place (i.e. /tmp/, or, probably preferrably, the cache directory). To identify that sort of dir in a cross-plattform manner, I propose using https://crates.io/crates/dirs