Adding the config tag for the OBMC experiment

obmc: We add an obmc prediction mode at superblock level.
When it is enabled, predictors of the above and left blocks
are used to refine the regular block-based motion compensation.

Change-Id: I6310104ea3dfece16d736351e368861471dd1c9b

configure

diff --git a/configure b/configure
index c93c22c..9769880 100755
--- a/configure
+++ b/configure
@@ -282,6 +282,7 @@
     ans
     loop_restoration
     ext_partition
+    obmc
 "
 CONFIG_LIST="
     dependency_tracking