Implement click listener for editor wrapper (#2230)

Summary:
**Summary**

As I understand the `onClick` listener on the wrapper `div` element is suppose to replicate native `textarea/input` behavior. But the actual value of `onClick` property was `undefined` and thus needed to be implemented

**Test Plan**

It's an example file, no test plan is required.
Pull Request resolved: #2230

Differential Revision: D18247675

Pulled By: mrkev

fbshipit-source-id: 88e7160776f66daf1033351e4c5f2f4cfb5d40f1

examples/draft-0-10-0/plaintext/plaintext.html

@@ -38,6 +38,7 @@
           this.onChange = (editorState) => this.setState({editorState});
           this.logState = () => console.log(this.state.editorState.toJS());
           this.setDomEditorRef = ref => this.domEditor = ref;
+          this.focus = () => this.domEditor.focus();
         }
 
         componentDidMount(){