heroku-like configuration handling for dokku

After much tweaking, I have something that works well (in my testing at least). The plugin has heroku-like usage, making switching between heroku and this plugin extremely easy.

I made the extra commits to make the default help output more sane. I also have some ideas on how that might change in the future - so help output is always formatted well - though it'll have to wait.

Original implementation available here: https://github.com/josegonzalez/dokku-config

Does not have pluginhooks, but then again, I don't think it's needed.

refs #55
superscedes #188