[API] Add hb_direction_from/to_string()

And hb-view --direction argument.

src/hb-common.h

diff --git a/src/hb-common.h b/src/hb-common.h
index c496dcf..e6f5f2b 100644
--- a/src/hb-common.h
+++ b/src/hb-common.h
@@ -92,6 +92,12 @@
   HB_DIRECTION_BTT
 } hb_direction_t;
 
+hb_direction_t
+hb_direction_from_string (const char *str);
+
+const char *
+hb_direction_to_string (hb_direction_t direction);
+
 #define HB_DIRECTION_IS_HORIZONTAL(dir)	((((unsigned int) (dir)) & ~1U) == 0)
 #define HB_DIRECTION_IS_VERTICAL(dir)	((((unsigned int) (dir)) & ~1U) == 2)
 #define HB_DIRECTION_IS_FORWARD(dir)	((((unsigned int) (dir)) & ~2U) == 0)