[Feature Request] Use $XDG_DATA_HOME

[columndeeply]

It would be great if Megabasterd checked if $XDG_DATA_HOME is set and if it is then used that path to store its database. I see there's a PR open from last year which implemented this #457

Was there anything stopping it from getting merged? If not I would be up to porting those changes to the newest version and opening a new PR.

EDIT: for the time being I've forked the project to add XDG support, I'll update that repo with the new releases: https://github.com/columndeeply/megabasterd.