Update xaringan_to_pdf(), package deps and path handling

[gadenbuie]

This is a bit of an entangled PR with a few relatively minor changes.

• I added myself as a contributor and moved chromote and pdftools to Suggests. chromote should definitely be in Suggests since it isn't on CRAN and isn't necessary for the rest of the package. I'm on the fence about pdftools, but it has a system dependency and is also only used by xaringan_to_pdf(), so I moved it too.
• I bumped the default delay to 1s in xaringan_to_pdf() because you mentioned animations may cause blank slides, although I found that the source of that problem is likely slide continuations. In any case, 1s is safer.
• I added include_partial_slides to xaringan_to_pdf(), which controls whether partial/continued slides are included in the PDF.
• I updated the other arguments to match the build_* functions, namely to use input and output_file
• We now automatically add file:// and I also added checks to make sure we're looking at xaringan slides before printing

And finally, I replaced DescTools with functions from fs. DescTools is a fairly big package with a reasonable number of dependencies. fs specializes in working with paths across all systems and is commonly used in the tidyverse/rmarkdown ecosystem so it's very likely to already be installed.

I'm pretty certain I didn't break anything with the last change, but please give it a look to make sure I wasn't missing a critical detail.

[gadenbuie]

I just pushed two more commits. 1c8a2f7 (#2) refactors build_all() to simplify the nested if...else structure. Again, let me know if I missed a detail there.

I also added the cli package to report build progress:

> build_all("inst/example/slides.Rmd", c("html", "gif"))
✓ Building slides.html from slides.Rmd ... done
✓ Building slides.pdf from slides.html ... done
✓ Building slides.gif from slides.pdf ... done

[jhelvy]

Awesome, thanks for these quick fixes! Particularly appreciate the {fs} suggestion - I wasn't super happy with the DescTools dependency but didn't know of a better solution for handling the file path problems.