Add countable interface

flag.go

@@ -124,6 +124,12 @@ type Flag interface {
 	GetValue() string
 }
 
+// Countable is an interface to enable detection of flag values which support
+// repetitive flags
+type Countable interface {
+	Count() int
+}
+
 func flagSet(name string, flags []Flag) (*flag.FlagSet, error) {
 	set := flag.NewFlagSet(name, flag.ContinueOnError)