Abstract `config.py` into stand-alone package

[mattjmcnaughton]

All of the configuration handling for sdep is non-specific to this given use-case (except for some constants which could easily be passed as configuration values). Given our configuration handling is stand-alone and reusable, we should package the configuration handling into a stand-alone package which we will host on pip. We should not spin out until configuration is relatively stable, otherwise it will be too much of a pain to have to update the pip package every time we want to make a change.