Minor change to allow access to current screen

Also a small robustness update.

Minor update to allow chained animations

Can now use the new property chain_animation_options with the switch_screens function to chain animations together.

Multi-Screen.js v1.2.0

• Addition of switch_screens(options) function for developers to use in
  their custom JS code, instead of being limited to links on pages.
• Addition of scrolling to the top of the page before animating (more
  smooth than snapping to the top). Time can be changed with
  data-ms-scroll-time, and default can be set with set_defaults(options)
  and set_default_time(time, type) with type ‘scroll’.