Enable ALT_INTRA experiment by default.

This tool was adopted in the AOM codec WG call on 03/03/2017.

Change-Id: I253e9cf0bd7471940f6bf5ef7f9cceb9ee72c708

configure

@@ -483,6 +483,7 @@ post_process_cmdline() {
     ! enabled ans && soft_enable daala_ec
     soft_enable ec_multisymbol
     soft_enable palette
+    soft_enable alt_intra
 
     # Fix up experiment dependencies
     enabled pvq && enable_feature ec_adapt