fix(extFlag): extFlag doesn't set default value prior to parsing

* `extFlag.PreParse()` will set the value to that of `flag.DefValue`.

Author: somebadcode

flag_ext.go

@@ -7,6 +7,10 @@ type extFlag struct {
 }
 
 func (e *extFlag) PreParse() error {
+	if e.f.DefValue != "" {
+		return e.Set("", e.f.DefValue)
+	}
+
 	return nil
 }