Variable processing in action

* Added processing for method.url for env vars. :clap: 
* Mocha tests accompanying the same 
* Have a few TODOs in place - do write your opinion on the queries asked.

To see this in action run this :pray: 

    ./bin/newman -u https://www.getpostman.com/collections/fc3f0598daaa5271e4f7 -e tests/data/environment.js