Missing alt= attribute in html vignettes

[MichaelChirico]

We have tooling that calls out .html images missing alt= tags. Three such examples are in the forcats vignette, e.g.:

https://github.com/tidyverse/forcats/blob/bd319e08dd1bbc4b292a06b0872203809101327e/vignettes/forcats.Rmd#L30-L34

Now that knitr supports adding an image alt, would you like me to flag these accessibility issues as I come across them?