Allow view providers to specify a createView factory method

If present, it will be called with the model object instead of
instantiating the view constructor directly and assigning a model on it.
This gives users more flexibility when constructing views.

spec/view-registry-spec.coffee

@@ -49,6 +49,24 @@ describe "ViewRegistry", ->
             expect(view2 instanceof TestView).toBe true
             expect(view2.model).toBe subclassModel
 
+        describe "when the provider has a createView method", ->
+          it "constructs a view element by calling the createView method with the model", ->
+            class TestModel
+            class TestView
+              setModel: (@model) ->
+
+            registry.addViewProvider
+              modelConstructor: TestModel
+              createView: (model) ->
+                view = new TestView
+                view.setModel(model)
+                view
+
+            model = new TestModel
+            view = registry.getView(model)
+            expect(view instanceof TestView).toBe true
+            expect(view.model).toBe model
+
       describe "when no view provider is registered for the object's constructor", ->
         describe "when the object has a .getViewClass() method", ->
           it "builds an instance of the view class with the model, then returns its root node with a __spacePenView property pointing at the view", ->

src/view-registry.coffee

@@ -29,8 +29,10 @@ class ViewRegistry
       object[0].__spacePenView ?= object
       object[0]
     else if provider = @findProvider(object)
-      element = new provider.viewConstructor
-      element.setModel(object)
+      element = provider.createView?(object)
+      unless element?
+        element = new provider.viewConstructor
+        element.setModel(object)
       element
     else if viewConstructor = object?.getViewClass?()
       view = new viewConstructor(object)