changed configure script to enable new experimental options

Added two experimental options to the configure script:
1. newnear:
   new scheme of doing mv encoding that include a motion vector from
   last frame in nearest and near mv search
2. mulcontext:
   tracks entropy context separately for regular frames and alt ref
   frames.

Change-Id: If6e0d5d593351707b497a26eb6a763e080f77e6f

configure

@@ -225,6 +225,8 @@ EXPERIMENT_LIST="
     i8x8
     qimode
     uvintra
+    newnear
+    mulcontext
 "
 CONFIG_LIST="
     external_build