Add hb_feature_to/from_string()

src/hb-common.h

diff --git a/src/hb-common.h b/src/hb-common.h
index 920bd32..cc221d3 100644
--- a/src/hb-common.h
+++ b/src/hb-common.h
@@ -95,10 +95,14 @@
 
 #define HB_TAG_NONE HB_TAG(0,0,0,0)
 
-/* len=-1 means str is NUL-terminated */
+/* len=-1 means str is NUL-terminated. */
 hb_tag_t
 hb_tag_from_string (const char *str, int len);
 
+/* buf should have 4 bytes. */
+void
+hb_tag_to_string (hb_tag_t tag, char *buf);
+
 
 /* hb_direction_t */