Support some form of environment variable config

[gtd]

This would make the gem usable in a large number of additional scenarios where we may not want config values in the repo:

• sensitive information such as passwords/tokens
• Heroku deployment
• Vagrant boxes where the same physical copy of files may be running in more than one environment simultaneously

I believe it can be done without encroaching on the existing functionality which makes LiteConfig so simple in practice, and is still widely useful for a large number of config settings which are not sensitive or specific to a precise environment.