Add chroma_sub8x8 experimental flag

Change-Id: I561f573114bc83d9dfd38a6ed78d8f07d429299b

configure

diff --git a/configure b/configure
index cb70ed2..e743800 100755
--- a/configure
+++ b/configure
@@ -294,6 +294,7 @@
     xiphrc
     cb4x4
     chroma_2x2
+    chroma_sub8x8
     frame_size
     delta_q
     ext_delta_q