Add the ability to register bindings after instantiation

[mberkom]

Added this.registerBindings function. It accepts an additional bindings object to be added to the binding provider's binding list.

This allows individual KO modules register their own bindings before applying them in their specific portion of the dom.

var bindings = { ... }

ko.bindingProvider.instance = new ko.classBindingProvider(bindings);

ko.bindingProvider.instance.registerBindings({
  ...
})

[rniemeyer]

While one could just extend ko.bindingProvider.instance.bindings directly, I think that it is fine to include the registerBindings API to simplify the call. Change is merged with specs/docs.