[charcoal/email] Allow email view controller to be customizable

[mcaskill]

The ViewableTrait allows one to customize the view controller used in rendering a template path identifier. By default, the view controller is the instance that implements the trait.

In Email, the accessor method is overridden—since its inception—which forces one to create a template controller for each template path identifier; without extending the Email class.

I don't know why this decision was made but it is annoying and restricts customization and flexibility.

For example, it would be practical to have a single project-side template controller and many different template files.