feat: Ability to call arbitrary command from a template

[mumoshu]

It would provide more flexibility to author your values.yaml files:

{{ exec "./myscript" | fromJson | toYaml }}

You can even exec an another template engine to dynamically generate your values.yaml. This way, our users won't need to wait until helmfile implements a bunch of template funcs to cover all the use-cases.

[mumoshu]

And it should take arbitrary number of args like {{ exec "./myscript" "--flag1" "val1" }}