Local mode for defaultOrKey func

Hi, we use consul-template in production and it fits perfectly our needs.

We want to have a single conf file for multiple environments (staging, prod, local, etc).

Problem with the local case, when developer needs to run the code during development/testing. We have defaults for every config key we load from consul in case if the key will not be available there. What we want is to have the possibility to render config without actual connection to Consul, and fill config with default values where possible.

Will you accept PR with such enhancement and if will which form will be acceptable? e.g. CLI flag `--offline`?

Thanks!

Start by locating the defaultOrKey function and the existing CLI entry points. Determine how an offline or local mode should avoid connecting to Consul while still rendering available defaults, and define completion as rendering the configuration successfully without an actual Consul connection.

Written by the indexing model from the issue text.