Specify File Extensions for --output-files

**Describe the problem/challenge you have**
Some people are opinionated that YAML files should end with a `.yaml` extension, however others prefer a `.yml` extension for brevity. Additionally, there may be complex cases where users are transitioning from other templating languages (e.g. Jinja2 `*.yaml.j2`) and would like the resultant output to be a proper YAML extension.

**Describe the solution you'd like**
Very simply, I think an argument such as `--output-extension` to be used with `--output-files` would suffice. The extension should be a proper extension as input by the user and begin with a `.`. Even more restrictive, users could be allowed to output with either `.yaml` or `.yml` file extensions.

**Anything else you would like to add:**
No additional info at this time.