#47 xdg config spec support

[Max]

Right now gnomint store default base in ~/.gnomint/default.gnomint
Would be much better to adhere to XDG specification by default.
The spec is available at http://standards.freedesktop.org/basedir-spec/latest/
Gnome software is already using it to some extent: https://wiki.gnome.org/Initiatives/GnomeGoals/XDGConfigFolders