Add TS definitions for onViewModeChange callback

Original commit by josefernand:
arqex@8ea6ea8

Author: simeg

DateTime.d.ts

@@ -88,6 +88,11 @@ declare namespace ReactDatetimeClass {
          input (a string).
          */
         onBlur?: EventOrValueHandler<FocusEvent<any>>;
+        /*
+         Callback trigger when the view mode changes. The callback receives the selected view mode
+         string ('years', 'months', 'days', 'time') as only parameter.
+         */
+        onViewModeChange?: (viewMode: string) => void;
         /*
          The default view to display when the picker is shown. ('years', 'months', 'days', 'time')
          */

react-datetime.d.ts

@@ -69,6 +69,11 @@ declare module ReactDatetime {
      input (a string).
      */
     onBlur?: (momentOrInputString : string|any) => void;
+    /*
+     Callback trigger when the view mode changes. The callback receives the selected view mode
+     string ('years', 'months', 'days', 'time') as only parameter.
+    */
+    onViewModeChange?: (viewMode: string) => void;
     /*
      The default view to display when the picker is shown. ('years', 'months', 'days', 'time')
      */

typings/react-datetime-tests.tsx

@@ -81,6 +81,9 @@ const TEST_INPUT_PROPS: JSX.Element = <ReactDatetime
 		onBlur={
 			(momentOrInputString:string) => {}
 		}
+		onViewModeChange={
+ 			(viewMode:string) => {}
+ 		}
  	/>;
 
 const TEST_EVENT_HANDLERS_WITH_MOMENT: JSX.Element = <ReactDatetime