add `df_print` argument

[dpprdan]

This adds the df_print argument to confluence_document() (cf. rmarkdown::output_format()), first and foremost to facilitate easy kable()/markdown output tables. df_print is passed down to rmarkdown::md_document().

Before I push this any further, would you be interested in this in general, @yutannihilation? Or is there a reason, this is not yet available?

Details:

• The "paged" method is not possible, because it is not a valid format for markdown documents.
• Arbitrary functions for printing data frames are not allowed ATM. They would have to generate a format that is markdown compatible and offer formatting that extents what is already possible with kable(). I would keep this out for now, but it could be added in a future iteration of the feature, of course.
• df_print should probably be added as an option to the addin as well.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[yutannihilation]

Thanks so much for catching! There's no reason other than I don't use df_print that much :)

Looks good in general.

[dpprdan]

I am trying to add df_print to the addin as well. However, the md_document is already rendered when the addin window opens. So df_print is already set at that point and there is no way to change it, AFAIU (short of rendering the whole document again, which seems ... not ideal). Or am I missing something?

[yutannihilation]

Ah, you are right.

[dpprdan]

I'll leave df_print out of the addin then, because this would involve a fairly profound change to the addin AFAIU (open the addin before rendering the preview, trigger the preview after setting options, publish after reviewing the preview).

[yutannihilation]

Sounds good to me. Most of the options of confluence_document() are used for controlling "how the document will be published," not customizing how the document is rendered, but df_print is the latter one. Sorry I didn't notice it at first.