テンプレート

hello.gohtml

下に入る
{{include "hoge.html"}}
上に入る

main.go

func main() {
	t := template.Must(template.New("hello.gohtml").Funcs(template.FuncMap{
		"include":	Include,
	}).ParseFiles("hello.gohtml"))
	err := t.Execute(os.Stdout, map[string]interface{}{})
	if err != nil {
		panic(err)
	}
}

func Include(url string) string  {
	return url + " is included"
}