This can be used as a placeholder for dynamically-generated
dropdownMenu, notificationItem,
messageItem, taskItem sidebarMenu,
or menuItem. If called directly, you must make sure to supply
the correct type of tag. It is simpler to use the wrapper functions if
present; for example, dropdownMenuOutput and
sidebarMenuOutput.
menuOutput(outputId, tag = tags$li)Output variable name.
A tag function, like tags$li or tags$ul.
renderMenu for the corresponding server side function
  and examples.
Other menu outputs: dropdownMenuOutput,
  menuItemOutput, renderMenu,
  sidebarMenuOutput