[matchbrackets addon] Clear matched brackets when the addon is turned off

Closes codemirror#4128

Author: marijnh

addon/edit/matchbrackets.js

@@ -102,8 +102,10 @@
   }
 
   CodeMirror.defineOption("matchBrackets", false, function(cm, val, old) {
-    if (old && old != CodeMirror.Init)
+    if (old && old != CodeMirror.Init) {
       cm.off("cursorActivity", doMatchBrackets);
+      if (currentlyHighlighted) {currentlyHighlighted(); currentlyHighlighted = null;}
+    }
     if (val) {
       cm.state.matchBrackets = typeof val == "object" ? val : {};
       cm.on("cursorActivity", doMatchBrackets);