Add reactive UILabel type with text property.

Summary: Closes #84

Reviewers: O2 Material Motion, O4 Material Apple platform reviewers, #material_motion, markwei

Reviewed By: O2 Material Motion, O4 Material Apple platform reviewers, #material_motion, markwei

Tags: #material_motion

Differential Revision: http://codereview.cc/D3209

src/reactivetypes/Reactive+UILabel.swift

@@ -0,0 +1,29 @@
+/*
+ Copyright 2017-present The Material Motion Authors. All Rights Reserved.
+
+ Licensed under the Apache License, Version 2.0 (the "License");
+ you may not use this file except in compliance with the License.
+ You may obtain a copy of the License at
+
+ http://www.apache.org/licenses/LICENSE-2.0
+
+ Unless required by applicable law or agreed to in writing, software
+ distributed under the License is distributed on an "AS IS" BASIS,
+ WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ See the License for the specific language governing permissions and
+ limitations under the License.
+ */
+
+import UIKit
+
+extension Reactive where O: UILabel {
+
+  public var text: ReactiveProperty<String> {
+    let view = _object
+    return _properties.named(#function) {
+      return .init("\(pretty(view)).\(#function)", initialValue: view.text ?? "") {
+        view.text = $0
+      }
+    }
+  }
+}