Json data source

Consul-template should be able to use json files as the basis for templates (`{{json "ami-id@local.json"}}`). Why?
- Testing templates without a consul installation
- Node-local data for use in scripts (ip addresses, zones, tags)
- Bootstrapping consul itself with node-local data

Still to be written: tests, docs for this functionality
Right, I didn't see that when I wrote that comment. However, I think we are adding special cases for when QM is nil, but I believe the API should always be "QM exists", even if that QM is "fake".