describe: add example

examples/common.h

@@ -46,6 +46,15 @@ struct args_info {
 };
 #define ARGS_INFO_INIT { argc, argv, 0 }
 
+/**
+ * Check current `args` entry against `opt` string.  If it matches
+ * exactly, take the next arg as a string; if it matches as a prefix with
+ * an equal sign, take the remainder as a string; if value not supplied, 
+ * default value `def` will be given. otherwise return 0.
+ */
+extern int optional_str_arg(
+	const char **out, struct args_info *args, const char *opt, const char *def);
+
 /**
  * Check current `args` entry against `opt` string.  If it matches
  * exactly, take the next arg as a string; if it matches as a prefix with