Expose more config options as settings or make them more easily filterable

[GaryJones]

While there are a lot of config options for reveal.js, it would be nice to have some of the more common ones be set on a per-slideshow basis, via settings on the Edit Slideshow screen. controlsTutorial, controlsLayout, slideNumber etc.

For the rest, making the values easy to filter (in PHP) instead of having to override the whole footer() callback would make it easier too. Any filter should have access to the current slideshow ID / name, so that it could be filtered on a per-slideshow basis as well.

[JonTheNiceGuy]

I'm starting to look at these options as part of my Hacktoberfest actions!