Add rake task to pull config vars from Heroku

Hi,

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 just modeled IO redirection commands as a rake task to get this done. 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!