Special value for environment source

[hazmat345]

• Yapcon version: 0.3.3
• Python version: 3.6.1
• Operating System: Centos 7

This arose out of #88:

Would it be possible to have a sort of dummy class defined in yapconf that can represent the environment source? That way instead of having to use a magic string you could pass an object.

# Current
spec.load_config('ENVIRONMENT')

# Proposed
spec.load_config(yapconf.env)