Add in knitr support

[csgillespie]

This issue #17 asked about knitr hooks for ragg. Having the following header:

ragg_png = function(..., dpi = 192) ragg::agg_png(..., res = dpi, units = 'in')
knitr::opts_chunk$set(dev = "ragg_png", fig.ext = "png")

makes the magic work.

• Is worth creating knitr_ragg functions inside ragg?
• If not, probably worth adding this to the README (or some other doc)
• I suppose it might be worth adding ragg to knitr, but this is up to @yihui

Happy to make PRs for points 1 & 2 above

[yutannihilation]

Maybe did you mention a wrong person...?

[csgillespie]

@yutannihilation Sorry. Autocomplete!