Add filter_intra experiment flag

Will break ext-intra into 2 experiments: ext-intra and filter-intra.

Change-Id: Ibf66e9b9d9307fd58a703eada9569b74d171434b
diff --git a/configure b/configure
index 6ea4063..15b4092 100755
--- a/configure
+++ b/configure
@@ -260,6 +260,7 @@
     dual_filter
     ext_tx
     ext_intra
+    filter_intra
     ext_inter
     ext_interp
     ext_refs