selenider 0.4.0

• find_each_element() and find_all_elements() are new functions that can
  be used to find elements using an element collection. They are designed to
  replace elem_flatmap(), which is now deprecated and defunct.
• element_list() is now deprecated in favour of
  as.list.selenider_elements().
• print_lazy() is now deprecated, as it offers little benefit and is complex
  to maintain.
• ChromoteSession$check_active() is used to make sure that the session is
  still active before performing an action.
• The timeout argument to open_url(), back(), forward(), and
  reload()/refresh() allows you to specify how long to wait for a page to
  load. They default to 60 seconds.