Add local template test variables as currently in the Postmark

[glebsonik]

In Postmark you can view and edit the template and edit test variables to see how the template works with actual data however, it does not work locally. I even tried (just out of curiosity) to edit preview.html and add some variables as in the code below

<script>
    var templates = {
      "test_mail_template": {
         // name, description, and guide keys
         // and then template variables
        "template_entity_1": { "template_variable_1": "some custom value 1" },
        "template_entity_2": { "template_variable_2": "some custom value 2", "template_variable_2": "some custom value 3" }
      }
</script>

I think this is very useful not to push some unfinished templates to postmark and have the ability to test templates with actual content on your local machine

Consider this as a feature request, thank you