Allow attribute mappings to set "canChangeModel" and "collection" on Scope.watch()

[jbdeboer]

We want to discourage calling Scope.watch() from within a directive. There are a few places that we call Scope.watch() from Angular core directives:

• ngClass
• ngModel

In both cases, we can't use attribute mappings (e.g. NgOneWay()) because we need to set either canChangeModel or collection on Scope.watch().

Attribute mappings should be able to set these parameters as well.

First, we need a syntax that directives can use.
Second, we need to implement that syntax.