ENH: Look for styles in ~/.config/matplotlib/stylelib for XDG compliance

Hi all,
I just noticed that the `styles` module currently only looks for user-specified styles in  `~/.matplotlib/stylelib`. I think it ought to also look in `~/.config/matplotlib/stylelib/` for consistency with `~/.config/matplotlib/matplotlibrc` and in order to comply with the [latest XDG standard](http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html).