Revising settings.php in order to add a local settings

I suggest that we revise settings.php in order to add a local settings file to allow for more flexible/independent local development. This was something we had discussed doing for the 2014 site, but it was too late in the game and we had too many people already setup locally to make the change. Would be nice if we could make this or a similar change early.
Quick note: `include` helps errors with some drush setups :)