Add CC sticky sidebar headers feature

This commit introduces a feature which makes the headers in the CC
sidebar remain visible (by sticking at the top) when scrolling down.

The idea was taken from the suggestion made at pekb/thread/60784834.

Change-Id: I6bbab04da855dbfa35ff8f8b1bbbc6d15946c36c
diff --git a/src/background.js b/src/background.js
index f83d807..315d6d8 100644
--- a/src/background.js
+++ b/src/background.js
@@ -13,7 +13,8 @@
   "batchduplicate": false,
   "escalatethreads": false,
   "movethreads": false,
-  "increasecontrast": false
+  "increasecontrast": false,
+  "stickysidebarheaders": false
 };
 
 function cleanUpOptions() {