Add cb4x4 experimental flag

Experiment on coding block at resolution of 4x4 block.

Change-Id: I6aa201038f00c590747d800edb0a3e76ab1a51e8

configure

diff --git a/configure b/configure
index 3c59c6e..df8364a 100755
--- a/configure
+++ b/configure
@@ -283,6 +283,7 @@
     alt_intra
     palette
     daala_ec
+    cb4x4
 "
 CONFIG_LIST="
     dependency_tracking