Add cb4x4 experimental flag

Experiment on coding block at resolution of 4x4 block.

Change-Id: I6aa201038f00c590747d800edb0a3e76ab1a51e8
diff --git a/configure b/configure
index 13d65cd..b6e94e6 100755
--- a/configure
+++ b/configure
@@ -265,6 +265,7 @@
     rans
     daala_ec
     parallel_deblocking
+    cb4x4
 "
 CONFIG_LIST="
     dependency_tracking