[WIP - incomplete] Postgres plugin

Not quite fully functional yet - I'm going for a 'local' postgres database installed on the docker host, instead of the external postgres server I used in my gist example, simply because I think this is going to be the more useful configuration for most people.

I think this is nearly complete, but have got an issue with making sure the containers are able to resolve the host's name properly (for a while I was struggling with them trying to talk to dokku.me after picking up the host's hostname..!) Somebody could probably fix this pretty quickly with a pair of fresh eyes.

Also needs documentation and a bit of tidying up of the scripts.

I'll work on a separate plugin for 'external postgres server' after this one.

Please note I've altered the Vagrantfile to make it easier to do repeated local iterations without pushing anything to github - can strip this out of the CR later if requested.