Add style and condition for multiple selection to modify example

Prior to #1877, it looked like selected features had a distinct style because they were being rendered on top of their unselected counterparts.  The change in 1e0d2af0f0aac7f87420cbd40e725669af6b87de gives the select interaction a distinct style to achieve the same.

The changes in #1877 also made it so the shift modifier didn't add to a selection by default.  The remaining changes here allow addition to and removal from the selection (with shift-click and any click respectively).