How to define a global directive that can work like ng-model?

How to define a global directive that can work like ng-model?

The most related question I can find on SO is "How to share a single
Directive across multiple modules in AngularJS". But I need something a
little different. I need something that can work more or less like
ng-model. The page does not need to have a named ng-app.
Here is the real problem I want to solve: I would like to define some
template as samples that shows good accessibility. For example, the label
for a required form field, should have arterisk (*). However, I don't want
to force the users to define a named module in the JS file. Instead, they
can just add ng-app(without name) to the html or body tag and include my
JS file.