[utils] add to feature array with printf syntax

src/features.h

@@ -12,6 +12,9 @@
 
 void feature_array_add(cstring_array *features, size_t count, ...);
 
+// Add feature using printf format
+void feature_array_add_printf(cstring_array *features, char *format, ...);
+
 // Add feature count to dictionary
 
 bool feature_counts_update(khash_t(str_uint32) *features, char *feature, int count);