Plugin config from environment

This change allows Dillinger's Server-side plugins to read their config from the environment, rather than from `plugin-config.json` files, which are difficult to manage when deploying to a git remote such as Heroku.

This also conforms more closely to the [12 Factor App](http://12factor.net/config).

The integrations with Google,  Dropbox and Github at [my hosted fork](http://subdillinger.herokuapp.com) are all using this method of configuration.