New feature - Experimental support for footnotes

[RLesur]

The CSS Generated Content for Paged Media Module WD specifies a footnote value for the CSS float property. For now, Paged.js does not support this feature.

The aim of this PR is to introduce an experimental support for footnotes. It is experimental because it relies on a hack. Fortunately, it works well with Chrome and Firefox.

The PR has two parts.

1. A Pandoc filter that inserts the footnotes contents in spans as described in example 7 of the GCPMD WD. This filter uses the blocks_to_inlines lua function that was recently exposed in Pandoc 2.2.3.

2. A Paged.js hook. As far as I have tested, it works well. The idea behind this hook is to enclose each footnote span in a right-floating handler with 100% width before the chunker defines the pages layout.
   Here's an example:
   
   With this handler, we keep a sufficient room for the footnotes on each page.
   After the chunker has finished its job, the hook builds a footnote area, inserts the footnotes in this area, and finally removes the handlers from the flow.
   I've tested different handlers, and the best results I have obtained is with p handlers.

I have to admit that this hook is very dirty but this is the only trick I've found and it seems intuitive to me.
I hope you will like it!

[yihui]

This is amazing! Thanks!

I have to admit that I don't fully understand the PR but the hack sounds reasonable to me. I just have one minor suggestion: how about changing the field name notes-as-footnotes to paged-footnotes? Note is a Pandoc concept, and I'm afraid average users don't understand it.

BTW, sometimes two footnotes won't work in RStudio Viewer, e.g.,

but as you said, they work well in Chrome, so this is no big deal to me.

[RLesur]

Glad you like this PR!
I've changed the field name.

The paged.js dev team told me they had similar ideas for footnotes. They will write a post soon.

About the RStudio Viewer, paged.js never worked for me in the viewer (I've updated to RStudio desktop Ubuntu 1.2.1070 today to check again). I got this kind of trouble when the room was insufficient (then footnotes overflow outside the page area). But if the same html file works in Chrome, I have no idea (maybe a too old version of chromium in RStudio 1.2?).
If you want to test, you can comment on these lines to see where the handlers are positioned.