countdown v0.3.0

• Added a warning state to the coundown timer that is enabled by setting
  warn_when = N. The warning class is applied to the timer for the last N
  seconds, and the colors of this state are configured with the arguments with
  color_warning_ prefix (thanks @hadley, #5).

• Added a new convenience function for full-screen (and stand-alone) countdown
  timers, countdown_fullscreen(). This function's defaults work best when
  called from RStudio, in xaringan slides it will still be necessary to fiddle
  with font_size, margin, and padding (thanks @hadley, #6).

• Add countdown_app() to launch an interactive Shiny app with a full-screen
  countdown timer.