Add compound-singleref (compound mode using single ref)

This new coding tool is targeted to add new inter compound modes using
single reference frame - initially NEAREST_NEARMV for single reference
prediction.

Change-Id: I415d40dc285a46a79680b4da0c3f2e2f3784b688

configure

@@ -315,6 +315,7 @@ EXPERIMENT_LIST="
     mv_compress
     frame_superres
     new_multisymbol
+    compound_singleref
 "
 CONFIG_LIST="
     dependency_tracking