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
- You probably don't need the `else` here, since the `t.Fatal` is like a `return` and would save you the indentation :smile:.
- Can you use the existing pattern in the other tests? I usually like to give as descriptive an error as possible like "expected x, but got y"