[Reland] Enable ALT_INTRA experiment by default.

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

Resubmitting, now that the log2() use has been removed:
https://aomedia-review.googlesource.com/#/c/8463/

Change-Id: Iec4cff00ea901ab3ed6ed703599ef41c3f860ca1

configure

@@ -476,6 +476,7 @@ 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