Add ext_intra experiment

Experiment for extended/extra intra prediction.

Change-Id: Icfeaeb62bafd69474302d2de36d42c6a077a46c2

configure

diff --git a/configure b/configure
index 8e3f24e..456f7c2 100755
--- a/configure
+++ b/configure
@@ -266,6 +266,7 @@
     emulate_hardware
     ext_tx
     misc_fixes
+    ext_intra
 "
 CONFIG_LIST="
     dependency_tracking