Support data-linking directly to class attribute

Hi Boris, never done this before so hope it's the right process. I found it frustrating that I couldn't control the presence of a css class on an element from my view model, so I modified the jsviews code to allow this.

class on an element to be controlled by a boolean (or "truthy") source
value, with syntax like:

`data-getfrom="class-myclass: [isMyClass]"`