Allow optional custom configs to be optional

This change tests first for the existence of the custom config variable and then the existence of each custom config file in turn. If found the directory path to append to that config file is changed to the custom path.

This allows fallback to the standard config when no custom config is present.

Maybe it would be even better to load both - the standard config, then the custom config, but this might be confusing for groupConfig.php. It could work using array merge, but it wouldn't be particularly clear or easy to explain.