Add DestinationPointer for flags generator

In this commit I added a DestinationPointer variable that should be set
if the `Destination` should be configured as a pointer for a specific
flag type. It is expected that Generic type which is an interface will
not be a pointer but a struct. Before this change the code compilation
was failing with `type *Generic is pointer to interface, not interface`.

See https://github.com/urfave/cli/issues/1441

cmd/urfave-cli-genflags/generated.gotmpl

@@ -17,7 +17,11 @@ type {{.TypeName}} struct {
   HasBeenSet bool
 
   Value {{if .ValuePointer}}*{{end}}{{.GoType}}
+<<<<<<< HEAD:cmd/urfave-cli-genflags/generated.gotmpl
   Destination {{if .NoDestinationPointer}}{{else}}*{{end}}{{.GoType}}
+=======
+  Destination {{if .DestinationPointer}}*{{end}}{{.GoType}}
+>>>>>>> Add DestinationPointer for flags generator:internal/genflags/generated.gotmpl
 
   Aliases []string
   EnvVars []string

cmd/urfave-cli-genflags/main_test.go

@@ -80,8 +80,9 @@ func genFlagType() *main.FlagType {
 					Type: "bool",
 				},
 			},
-			TypeName:     "YeOldeBlerfFlag",
-			ValuePointer: true,
+			TypeName:           "YeOldeBlerfFlag",
+			ValuePointer:       true,
+			DestinationPointer: true,
 		},
 	}
 }
@@ -115,6 +116,21 @@ func TestFlagType_ValuePointer(t *testing.T) {
 	}
 }
 
+func TestFlagType_DestinationPointer(t *testing.T) {
+	ft := genFlagType()
+
+	if !ft.DestinationPointer() {
+		t.Errorf("expected DestinationPointer to be true")
+		return
+	}
+
+	ft.Config = nil
+
+	if ft.DestinationPointer() {
+		t.Errorf("expected DestinationPointer to be false")
+	}
+}
+
 func TestFlagType_GenerateFmtStringerInterface(t *testing.T) {
 	ft := genFlagType()