Add chroma_2x2 experimental flag

Support 2x2 chroma block size.

Change-Id: I2e06b26b23bc2e2286825c1acd5a607b47b154d7

configure

diff --git a/configure b/configure
index 13c2526..5bc8657 100755
--- a/configure
+++ b/configure
@@ -288,6 +288,7 @@
     pvq
     xiphrc
     cb4x4
+    chroma_2x2
     frame_size
     delta_q
     adapt_scan