XDG directory spec

Fix for #109

This is a full implementation that uses $XDG_CONFIG_HOME and $XDG_CONFIG_DIRS.
The rationale is that since CONFIG_HOME corresponds to /etc and DATA_HOME corresponds to /usr/share, everything should go in CONFIG_HOME.

This currently breaks support on Windows and Mac, so I'd like some pointers on what the config directories on those platforms are.
According to the spec, you're supposed to use the fallback if the env var is present but empty. So you may want something along the lines of `if (tmp && *tmp)` here. Otherwise you'd end up using `/mpv` in that case.