Add configuration items to implement templates that execute light and dark separately.

[acdcbyl]

When I use matugen to generate colours, I sometimes need to use separate templates for light and dark, and I'd like to be able to add an option to the config file (e.g. mode-run) to run the templates separately. I'm not sure if this feature is necessary, but it does improve the level of customisation. All in all, thanks to the developers!☺️