add new feature: pagebreak for every format

[cderv]

This PR implements a new feature that will close #1610 and which is based on pandoc lua filter called pagebreak.

With this PR, it is now possible to use \newpage or \pagebreak in any Rmarkdown and will create a pagebreak in the correct output syntax for word_document(), html_document() and odt_document(). For pdf_document, those are already working latex command.
Currently, a vignette in this PR explains how it works.

This specific pagebreak lua filter is not from me and was taken from https://github.com/pandoc/lua-filters/tree/master/pagebreak. The file can be used as is (with a PR I made today there).
To use this file, the copyright notice is left in the file and I added the two authors for copyright in DESCRIPTION - I think this is how it should be done.

Also, lua filters are already used in pagedown but not in rmarkdown. This PR adds a small helper to include some lua filters with Rmarkdown. I think lua filter could be a really could way to add more features for several formats easily, and maybe also in other framework (I think of bookdown for example). I did not export the function and reuse a similar one from pagedown, adding a test for pandoc 2.0 as it works for 2+.

This means this pagebreak feature only work with pandoc >= 2.0. With earlier version, nothing change.

As I said above, I documented in a vignette. I don't know if this is the best place. If this feature makes it to rmarkdown, a part in a future book could also be an idea to documented it (and maybe makes the documentation shorter in the package).

Finally, I added some tests but only for html and latex output. I am not sure how to test word or odt easily. So basically, this test that the filter is correctly found and activated.

This PR is now functional. However, some things we could change on how it works:

• Currently, the lua filter is always activated if pandoc >= 2.0, even if not used. This has no impact on the result but may have impact on rendering time for big documents. (just a guess)
  I hesitated on making it optional.

• If you think it is better to make it optional, I see two ways:

  1. add an argument to activate the feature manually in each output format (like toc). It could be pagebreak: false by default and pagebreak: true to activate. As a user, I am not sure this is a good idea. If I wrote \pagebreak or \newpage, I would like it to work without doing nothing.
  2. do like in citeproc_required() and detect if a keyword is used in the input_lines. This would be detecting \newpage or \pagebreak when output is html, word or odt, then activating the filter only if found.

• I added the filter also for pdf_document() but as it has no impact, I believe it could be remove from pdf_document() in rmarkdown, even if the lua filter can handle tex output.

I hope you'll find this as interesting as I am.

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This change is 

[yihui]

Looks good to me. If you don't have further changes to make, I'll merge it. Thank you!

BTW, this will definitely be a good topic for the R Markdown Cookbook.

[cderv]

Thanks ! I just need to see if we promote break-* over page-break-*. I am unsure but @RLesur is the css expert here...

BTW, this will definitely be a good topic for the R Markdown Cookbook.

I agree. I was waiting to know if it would be accepted. I wrote the vignette with the book in mind. I'll add it in the book: Lua Topic (rstudio/rmarkdown-cookbook#170) + this new features as example.

[cderv]

@RLesur, do you find it better now ?

I did not change the page-break-after rule used as default in the lua filter but I use break-after in the custom class example and add link to caniuse.

If it is ok with you, then @yihui can merge.

[RLesur]

@cderv this is perfect! Just another suggestion, maybe the vignette could mention https://github.com/crsh/rmdfiltr

[cderv]

😮 I did not know that one ! Nice !
It does not have lot of filters right now but it could be the place to add more if not included as default feature inside rmarkdown.

We can definitely add mention to this in the vignette and in the book.

[crsh]

I'd be more than happy to collect additional filters in rmdfiltr. PRs are always welcome.

[yihui]

Thanks everyone!

[cderv]

@yihui I have just added reference to the two packages known relating to lua filters as suggested by @RLesur. If still ok with you, you can now merge. ☺️