Feature-request: one-shot exec mode support

To be able to notify some external services it will be great to have one-shot exec mode.

Issues of using template commands for this purpose:

- It is per-template - there is no way to know when all templates are done.
- It is not executed if no changes are required(file already exists)

Related thread:
https://github.com/hashicorp/consul-template/issues/1011

Review the related thread in issue #1011 first, then trace the existing template command and execution entry points in the repository. Define how one-shot execution detects completion of all templates and behaves when no files change; the feature is done when both notification cases are covered.

Written by the indexing model from the issue text.