Instead make it require .gotmpl extension to enable templating so that it looks like helmfile.yaml.gotmpl or helmfile.gotmpl.

As originally described in #584, the benefit of helmfile.yaml being not a go template is that you don't need "ugly"(some people think so) backticks to escape release template expressions #439.

And it would make helmfile slightly easier to integrate with tools like kapitan and jsonnet, that allows you to leverage richer language features to generate helmfile configs.

Note that #439 are enabled regardless of the file extensions. They're already vanilla YAML string literals containing go template expressions, that aren't handled by the whole-file templating.

Extracted from #584 (comment)