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
I usually like to use `t.Fatal(err)` here, that way the test shows the actual error.