Customise Mousetrap library useCapture flag

To be able to stop the propagation of events to the website but still
capture them in Vimary we need to listen to the keyEvents using the
onCapture flag. Something that has not yet been implemented in
Mousetrap (see ccampbell/mousetrap#400).

Vimari Extension/js/lib/mousetrap.js

@@ -1,3 +1,6 @@
+// Custom behaviour for Vimari implemented at line 185. Be aware of this
+// when upgrading mousetrap.
+
 /*global define:false */
 /**
  * Copyright 2012-2017 Craig Campbell
@@ -175,7 +178,11 @@
      */
     function _addEvent(object, type, callback) {
         if (object.addEventListener) {
-            object.addEventListener(type, callback, false);
+            // CUSTOMISATION:
+            // We set the useCapture to true such that events
+            // are handled before being dispatched to any EventTarget beneath
+            // it in the DOM tree.
+            object.addEventListener(type, callback, true);
             return;
         }