Re-introduce ctrl, shift keys as link hint mode modifiers

This fixes philc#1096.

content_scripts/link_hints.coffee

@@ -180,13 +180,25 @@ LinkHints =
       if event.keyCode == keyCodes.shiftKey
         @setOpenLinkMode(if @mode is OPEN_IN_CURRENT_TAB then OPEN_IN_NEW_BG_TAB else OPEN_IN_CURRENT_TAB)
 
+        handlerStack.push
+          keyup: (event) =>
+            return if (event.keyCode != keyCodes.shiftKey)
+            @setOpenLinkMode prev_mode if @isActive
+            handlerStack.remove()
+
       else # event.keyCode == keyCodes.ctrlKey
         @setOpenLinkMode(if @mode is OPEN_IN_NEW_FG_TAB then OPEN_IN_NEW_BG_TAB else OPEN_IN_NEW_FG_TAB)
 
+        handlerStack.push
+          keyup: (event) =>
+            return if (event.keyCode != keyCodes.ctrlKey)
+            @setOpenLinkMode prev_mode if @isActive
+            handlerStack.remove()
+
     # TODO(philc): Ignore keys that have modifiers.
     if (KeyboardUtils.isEscape(event))
       @deactivateMode()
-    else
+    else if (event.keyCode != keyCodes.shiftKey and event.keyCode != keyCodes.ctrlKey)
       keyResult = @getMarkerMatcher().matchHintsByKey(hintMarkers, event)
       linksMatched = keyResult.linksMatched
       delay = keyResult.delay ? 0