Add support for multiple slideshows per page

[cosmiccoincidence]

Currently, if you try to add more than one slideshow per page, it will break both slideshows and neither will work.

Add the functional capabilities to support more than one slideshow per blog.

Files related to the slideshow:
_layouts\default.html
_includes\slideshow.html
_sass\slideshow.scss

The first slideshow was added to: _posts\2020-05-30-20.05.md

And you can see it in action near the bottom of this page: https://ss3d.space/2020/04/03/20.03.html