FIx: Allow ext flags to be opt-out by default rather than opt-in

testdata/godoc-v2.x.txt

@@ -324,6 +324,9 @@ type App struct {
 	UseShortOptionHandling bool
 	// Enable suggestions for commands and flags
 	Suggest bool
+	// Allows global flags set by libraries which use flag.XXXVar(...) directly
+	// to be parsed through this library
+	AllowExtFlags bool
 
 	// Has unexported fields.
 }