Add more RISE config options to SlidesExporter

[lkeegan]

• reveal_overlay
• reveal_header
• reveal_backimage
• reveal_footer

These options are documented here: https://rise.readthedocs.io/en/latest/customize.html#decorating-all-slides Styling and ids of these elements follow RISE: https://github.com/damianavila/RISE/blob/master/classic/rise/static/main.js#L446-L470

Also update the reveal config options using the reveal dict in the notebook metadata if provided.

[lkeegan]

Example of use:

jupyter nbconvert tests/files/notebook1.ipynb --to slides --execute --SlidesExporter.reveal_header="Here is the header" --SlidesExporter.reveal_backimage="https://images.pexels.com/photos/255379/pexels-photo-255379.jpeg" --SlidesExporter.reveal_footer="Here is the footer"