text: Title: Nice to have: label property for dccRadioItems
  Body: Creating a radio button with:
  
  ```
  radioButton <- dccRadioItems(
    id = "radiobutton-selector",
    options = list(
    list("label" = "ggplotly", "value" = "ggplotly"),
    list("label" = "plotly", "value" = "plotly")
  ), value = "ggplotly")
  ```
  results in:
  <img width="92" alt="Screen Shot 2019-09-30 at 3 05 47 PM" src="https://user-images.githubusercontent.com/20918264/65908630-d2a1e100-e394-11e9-9bac-89e8ac0bc369.png">
  
  However, having a `label` or `title` property could save user from creating an additional component and aligning it with the radioselector. 
  ```
  radioButton <- dccRadioItems(
    id = "radiobutton-selector",
    *label = "Plot Type"*
    options = list(
    list("label" = "ggplotly", "value" = "ggplotly"),
    list("label" = "plotly", "value" = "plotly")
  ), value = "ggplotly")
  ```
  
  desired result:
  <img width="97" alt="Screen Shot 2019-09-30 at 3 08 00 PM" src="https://user-images.githubusercontent.com/20918264/65909006-90c56a80-e395-11e9-804b-277f083fb16d.png">
  
  
  
  
  
  
  
  