Add "Chart Categories" in YAML

[jwildfire]

Add a new optional "Chart Category" parameter that breaks up the chart dropdown in the nav using a label for category.

Implementation should be easy enough. Here's some possibly useful code from this example:

navbarPage("App Title",
  tabPanel("Plot"),
  navbarMenu("More",,
    "Chart Category #1",
    tabPanel("chart1"),
    tabPanel("chart2"),
    "Chart Category #3",
    tabPanel("chart3"),
    tabPanel("chart3")
  )
)