Add rake task to pull config vars from Heroku

In one of your open issues someone was asking if Figaro could "pull" environment variables from Heroku into application.yml. I decided to try my hand at building this feature!

I wrote a rake task that redirects the output of heroku:config to application.yml. Unfortunately it wouldn't be portable for Windows users because it had to implement pattern matching to make sure that only key/value pairs were being redirected to the file. YAML complains, otherwise. I understand if that's a deal-breaker.

I'd like to hear your thoughts about the implementation - if you hate it, I can try again!