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
Ah :smile:. I was actually thinking about this while reading the `File.Fetch()` function. Good idea, but I think we are going to need to return a queryMeta structure with those values set. As it is now, I think this is running like a million times for FileDependency (since there's no "blocking"). What do you think about setting the lastIndex to something based off the shasum of the file contents (just as an example)?