resolves #61 allow slide selector to be specified

[mojavelinux]

This patch does introduce some cyclomatic complexity and a slight increase in file size, but I think it completes the picture.

For complaint browsers, the ideal arguments are as follows:

bespoke.from({ parent: 'article', slides: ':scope > section' }, [(plugins)]);

However, :scope is still a fairly recent addition. Of course, this patch provides plenty of other ways to arrive there, including:

bespoke.from({ parent: 'article', slides: 'article > section' }, [(plugins)]);

or even :root article > section to break out of the context.

[markdalgleish]

Thanks for the pull request! I'll bump the version once we've merged your other pull request.

[mojavelinux]

Super! Thanks @markdalgleish!