purple: add $plugindir/purple to the plugin search path
This allows adding bitlbee-specific purple plugins in a directory
controlled by the user who starts bitlbee (as it can be defined in
bitlbee.conf, PluginDir).
Pidgin and finch have something similar allowing users to place plugins
in ~/.purple/plugins:
path = g_build_filename(purple_user_dir(), "plugins", NULL);
The direct equivalent would be to use our config dir, but i'd rather not
put executable modules there.
