Add ability to add onMouseDown extra functionality

Author: cjblomqvist

lib/draggable.js

@@ -239,7 +239,13 @@ module.exports = React.createClass({
 		 *	}
 		 * ```
 		 */
-		onStop: React.PropTypes.func
+		onStop: React.PropTypes.func,
+
+		/**
+		 * A workaround option which can be passed if onMouseDown needs to be accessed, since it'll always be blocked (due to that there's internal use of onMouseDown)
+		 *
+		 */
+		onMouseDown: React.PropTypes.func
 	},
 
 	componentWillUnmount: function() {
@@ -261,7 +267,8 @@ module.exports = React.createClass({
 			zIndex: NaN,
 			onStart: emptyFunction,
 			onDrag: emptyFunction,
-			onStop: emptyFunction
+			onStop: emptyFunction,
+			onMouseDown: emptyFunction
 		};
 	},
 
@@ -282,6 +289,9 @@ module.exports = React.createClass({
 	},
 
 	handleMouseDown: function (e) {
+		// Make it possible to attach event handlers on top of this one
+		this.props.onMouseDown(e);
+
 		var node = this.getDOMNode();
 
 		// Short circuit if handle or cancel prop was provided and selector doesn't match