fix(AllowLeadingHyphen): fixes a bug where valid args aren't recognized with this setting

Closes #588

Author: kbknapp

src/app/parser.rs

@@ -753,7 +753,7 @@ impl<'a, 'b> Parser<'a, 'b>
                     name: sc_name,
                     matches: sc_m.into(),
                 });
-            } else {
+            } else if !self.settings.is_set(AppSettings::AllowLeadingHyphen) {
                 return Err(Error::unknown_argument(&*arg_os.to_string_lossy(),
                                                    "",
                                                    &*self.create_current_usage(matcher),