Add `collapsed` argument to dashboardSidebar()

[bborgesr]

This allows the sidebar to start off collapsed (fixes #73).

Should the argument name be "collapsed" or "startCollapsed" (or something else)?

Example app:

library(shinydashboard)
library(shiny)

shinyApp(
  ui = dashboardPage(
    dashboardHeader(),
    dashboardSidebar(collapsed = TRUE),
    dashboardBody()
  ),
  server = function(input, output, session) {}
)