Add script for performance-guided optimization. Ignore pgo files.

Thanks to @gwern & @FeepingCreature for showing this to me.

.gitignore

@@ -1,4 +1,5 @@
 *.dSYM
+*.gcda
 *.o
 *.plist
 .deps

pgo.sh

@@ -0,0 +1,12 @@
+#!/bin/sh
+
+set -e
+cd "$(dirname "$0")"
+
+make clean
+CFLAGS="$CFLAGS -fprofile-generate"
+./build.sh
+./ag example ..
+make clean
+CFLAGS="$CFLAGS -fprofile-use"
+./build.sh