Improve slides

[chrisjsewell]

• There is another package nbpresent so maybe I should change the command name to avoid conflict.
• it outputs "output_type": "stream" in a weird position (could be a problem for html_ipypublish as well)
• remove specific first_cell tag and handle this in the jinja tamplate
• title page from latex_docs information
• add fragments options
  NB: this could be an issue with the SplitOutputs preprocessor
• better control of individual slide content (currently just splits out headers and sub-headers)
• auto-number headings
  have this working for h1 but always seems to mess up when attempting lower levels like this
• outline of sections on left and/or first slide, with faded text for everything but current section
  maybe this: https://github.com/frederickf/presentable/wiki/For-reveal.js
• captions from cells
• code cells (and captions)
• figure/table/equation numbering (that matches references)
• links in slides don't work (but do in normal html)
  asked about it here: https://stackoverflow.com/questions/45160379/how-to-make-internal-links-work-in-reveal-js-slides
• handle adding reveal.js plugins (inject them into the code or copy them next to output)
  do you have to specify dependancy src as file path?
• nicer css?

There is also an open ticket in jupyter/nbconvert#612