Support _ in feature ids
diff --git a/src/hb-common.cc b/src/hb-common.cc
index a02d325..6d12c09 100644
--- a/src/hb-common.cc
+++ b/src/hb-common.cc
@@ -847,7 +847,7 @@
   }
 
   const char *p = *pp;
-  while (*pp < end && ISALNUM(**pp))
+  while (*pp < end && (ISALNUM(**pp) || **pp == '_'))
     (*pp)++;
 
   if (p == *pp || *pp - p > 4)