Load variables from yaml and / or json file

[krak3n]

This idea, floated by @mikeywaites would allow users to define project variables in a yaml or json file without the need for command line arguments. When dealing with many variables in a template this could help keep things clear and speed things up.

[mikeywaites]

Up for taking this one on dude. You had any thoughts on how you might tackle it? If i remember our conversation correctly we had some ideas around a cascade in which vars could be overridden by passing them as command line options etc. Might be cool to allow external requests for yaml, json files to so gists could be used for example?