Install plugins to /usr/share rather than /usr/lib since they're
arch-independent. Many precedents can be found for scripted plugins
in /usr/share and fewer for /usr/lib.
Look for plugins in all XDG data directories and repurpose
the "plugin_dir" setting to override this behaviour.
This adds some complexity to the bot but unifies the project.
It might make sense to remove the "plugin_dir" setting.
