Add ability to restore bookmarked tabs

[wch]

This closes #152.

It adds the ability to restore selected menuItems and menuSubItems in the sidebar if they have a corresponding tabItem. However, menuSubItems still don't have the ability to start expanded at this time, so although the tab content will be visible, the menuSubItem itself will not be.

Simple test app:

library(shiny)

ui <- function(request) {
  sidebar <- dashboardSidebar(
    sidebarMenu(id = "sidebarmenu",
      menuItem("Dashboard", tabName = "dashboard", icon = icon("dashboard")),
      menuItem("Subitems",
        menuSubItem("Subitem 1", "subitem1"),
        menuSubItem("Subitem 2", "subitem2")
      ),
      menuItem("Widgets", icon = icon("th"), tabName = "widgets",
               badgeLabel = "new", badgeColor = "green"),
      bookmarkButton()
    )
  )

  body <- dashboardBody(
    tabItems(
      tabItem(tabName = "dashboard",
        h2("Dashboard tab content")
      ),
      tabItem(tabName = "subitem1",
        h2("Subitem 1 tab content")
      ),
      tabItem(tabName = "subitem2",
        h2("Subitem 2 tab content")
      ),

      tabItem(tabName = "widgets",
        h2("Widgets tab content")
      )
    )
  )

  dashboardPage(
    dashboardHeader(title = "Simple tabs"),
    sidebar,
    body
  )
}
server <- function(input, output) { }

shinyApp(ui, server, enableBookmarking = "url")