Add ability to disable custom notification individually

[stevebrownlee]

When working in a team that has custom notifications for some tasks, e.g.

grunt.registerTask("purge", ["clean", "notify:cleaned"]);
grunt.registerTask("default", ["lint", "style", "copy", "notify:build"]);

Some team members may not want to see some, or all, notifications, so this adds the ability to turn off notifications individually.

module.exports = {
  cleaned: {
    options: {
      enabled: false,
      title: "My Project",
      message: "Project assets cleaned"
    }
  },
  build: {
    options: {
      enabled: true,
      title: "My Project",
      message: "Project successfully built"
    }
  }
};