diff: Add an option to control syntax aliases

kittens/diff/diff.conf

@@ -1,3 +1,11 @@
+# vim:fileencoding=utf-8:ft=conf
+
+# File extension aliases for syntax highlight
+# For example, to syntax highlight file.xyz as
+# file.abc use a setting of xyz:abc
+syntax_aliases pyj:py recipe:py
+
+# Colors
 foreground            black
 background            white
 title_fg              black