line break mode for ios

Summary:
What do you think is ```lineBreakMode``` a good name? For android it is called ```ellipsize```.

<img src="https://cloud.githubusercontent.com/assets/1488195/15628555/7372f8d0-250c-11e6-8919-722f28a38d60.png"" width="300" />
Closes https://github.com/facebook/react-native/pull/7819

Differential Revision: D3417256

fbshipit-source-id: 189441a23ff554bf7f6d67fa8510959351e9e5cc

Libraries/Text/Text.js

@@ -29,6 +29,7 @@ const viewConfig = {
   validAttributes: merge(ReactNativeViewAttributes.UIView, {
     isHighlighted: true,
     numberOfLines: true,
+    lineBreakMode: true,
     allowFontScaling: true,
   }),
   uiViewClassName: 'RCTText',
@@ -69,6 +70,11 @@ const viewConfig = {
 
 const Text = React.createClass({
   propTypes: {
+    /**
+     * Line Break mode. Works only with numberOfLines.
+     * clip is working only for iOS
+     */
+    lineBreakMode: React.PropTypes.oneOf(['head', 'middle', 'tail', 'clip']),
     /**
      * Used to truncate the text with an ellipsis after computing the text
      * layout, including line wrapping, such that the total number of lines
@@ -110,6 +116,7 @@ const Text = React.createClass({
     return {
       accessible: true,
       allowFontScaling: true,
+      lineBreakMode: 'tail',
     };
   },
   getInitialState: function(): Object {