Load Environment Variables specified in config file

It is an often [frowned upon](http://12factor.net/config) practice to hard code sensitive information in files checked into your codebase. For this reason, it would be handy if we could specify the name of an environment variable to use as the value in a configuration entry.

This PR implements that feature, which looks for a configuration value like `{ "ENV" : "env_var_name_here" }`, and then grabs the value with `process.env`.