adds input event, also fixes changed event docs

Author: futuremarc

src/core/p5.Element.js

@@ -241,15 +241,28 @@ p5.Element.prototype.mouseOver = function (fxn) {
  * This can be used to attach an element specific event listener.
  *
  * @method changed
- * @param  {Function} fxn function to be fired when mouse is
- *                    moved over the element.
+ * @param  {Function} fxn function to be fired when the value of an element changes.
  * @return {p5.Element}
  */
 p5.Element.prototype.changed = function (fxn) {
   attachListener('change', fxn, this);
   return this;
 };
 
+/**
+ * The .input() function is called when any user input is detected with an element.
+ * The input event is often used to detect keystrokes in a input element,
+ * or changes on a slider element. This can be used to attach an element specific event listener.
+ *
+ * @method input
+ * @param  {Function} fxn function to be fired on user input.
+ * @return {p5.Element}
+ */
+p5.Element.prototype.input = function (fxn) {
+  attachListener('input', fxn, this);
+  return this;
+};
+
 /**
  * The .mouseOut() function is called once after every time a
  * mouse moves off the element. This can be used to attach an