Allow distill vignettes

[mpadge]

Current vignette test is:

pkgcheck/R/check-vignette.R

Line 24 in e499f62

format <- rmarkdown::default_output_format (f)$name

That works for rendering vignettes with rmarkdown, but not with distill. This leads to failures in ropensci/software-review#623 (comment).

@AAoritz Sorry for the inconvenience; we'll enable your package to pass that check asap.

@maelle Do you want to tackle this one? It'll just require an extra if clause to detect distill instead of rmarkdown, and perform appropriate checks for that.

[mpadge]

@maelle All good; was an easy fix