Changed XDG data dir to '~/.local/share'

If `XDG_DATA_HOME` isn't set, then xcmenu's directory for history storage is created in `~/.local`. Setting it to `~/.local/share` is the correct way as stated here:
>$XDG_DATA_HOME defines the base directory relative to which user specific data files should be stored. If $XDG_DATA_HOME is either not set or empty, a default equal to $HOME/.local/share should be used.

http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html

Besides the spec, the main thing is that it's just more organized. By the way, I tested this out and it seemed to work fine, but given how small the change is I might have forgotten something.