development: add option to launch story at specific slide

[petervidos]

A story should start at the specified slide when it loads. Slides are counted automatically, beginning with 1.

[petervidos]

Also add the option to use negative numbers with the following logic:
-1 - last slide
-2 - the slide before the last slide
and so on.

[simzer]

Use case:

• when developing a story, it is cumbersome to step again and again to a specific slide.

Solutions:

• startslide attribute to the HTMLElement
• URL # - change/update - this 2nd solution could be enabled when needed, but best to leave it switch off by default, since the URL fragment can be used by the user for different purposes.