Add ability to cancel adding link using Escape

Add a keydown listener on component, which if you hit Escape while you
are editing the link will stop without canceling out of the block. If
you hit escape again, it will cancel the block.

blocks/editable/format-toolbar.js

@@ -2,6 +2,7 @@
  * WordPress dependencies
  */
 import { IconButton, Toolbar } from 'components';
+import { ESCAPE } from 'utils/keycodes';
 
 const FORMATTING_CONTROLS = [
 	{
@@ -36,6 +37,11 @@ class FormatToolbar extends wp.element.Component {
 		this.dropLink = this.dropLink.bind( this );
 		this.submitLink = this.submitLink.bind( this );
 		this.updateLinkValue = this.updateLinkValue.bind( this );
+		this.onKeyDown = this.onKeyDown.bind( this );
+	}
+
+	componentDidMount() {
+		document.addEventListener( 'keydown', this.onKeyDown );
 	}
 
 	componentWillUnmout() {
@@ -44,6 +50,19 @@ class FormatToolbar extends wp.element.Component {
 		}
 	}
 
+	onKeyDown( keydown ) {
+		switch ( keydown.keyCode ) {
+			case ESCAPE:
+				if ( this.state.isEditingLink ) {
+					keydown.preventDefault();
+					this.dropLink();
+				}
+				break;
+			default :
+				break;
+		}
+	}
+
 	componentWillReceiveProps( nextProps ) {
 		const newState = {
 			linkValue: nextProps.formats.link ? nextProps.formats.link.value : '',