Allow HTML articles without abstracts

[zeileis]

We tried to convert our "Changes on CRAN" report that is published in every R Journal issue from .tex to .Rmd. However, two aspects that we have always used are not yet supported in .Rmd specifically in HTML output:

1. There is currently no support of a subtitle. For PDF output this was straightforward to add in the LaTeX template, see PR support setting subtitle from YAML in LaTeX template #58. However, for HTML output I was not able to add this because I don't know distill well enough.
2. The abstract cannot be empty. For PDF output this is already supported and the abstract is omitted. But for HTML output it is not and a default abstract is generated.

Let me know if you need further details or I can help in any way. If you can give me some guidance on how to implement these two features, I'm willing to try to implement them and make a PR.

[zeileis]

Thanks for the update on this. Which packages do I need to install to make this work? I tried to install the GitHub versions of rjournal/rjtools and rstudio/distill. And the PDF works with both subtitle and no abstract now - but the HTML still has no subtitle and a default abstract.

[dicook]

Oh, I was a bit too zealous.

[zeileis]

Thanks.

[mitchelloharawild]

Patch fixed in f2d810c

[mitchelloharawild]

Clarification: the patch fixes HTML behaviour with subtitles

An abstract is currently required (not explicitly, but a default one is produced if the article doesn't provide one).