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.:

forcats/vignettes/forcats.Rmd

Lines 30 to 34 in bd319e0

	```{r initial-plot}
	ggplot(starwars, aes(x = hair_color)) +
	geom_bar() +
	coord_flip()
	```

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

[hadley]

@MichaelChirico yes please!