Adding placeholders for new expts to be added

Change-Id: I38952cd55b91f35e5db45bc8e6a20ef25069c464
--ext-refs:   extended references - for multi-ref in nextgen
--ext-inter:  extended inter - for new_inter/copy_mode in nextgen
--ext-interp: for new interpolation

configure

@@ -268,6 +268,9 @@ EXPERIMENT_LIST="
     ext_tx
     misc_fixes
     ext_intra
+    ext_inter
+    ext_interp
+    ext_refs
 "
 CONFIG_LIST="
     dependency_tracking