This PR adds a new parameter preset for ggsave() that allows plot width and height (and units) to be specified at once. It recognizes common paper sizes (such as "a4" and "letter"), common screen resolutions ("4k"), and resolution specifications such as "1920x1080".

All presets default to landscape orientation, and portrait can be specified by appending an "r" (for rotated) to the end of the string. This is similar to the paper option for grDevices::pdf(), but with landscape being the default as it's probably a more common choice for plots.

(Naturally this new parameter could also be called paper instead of preset, but 1) that felt a bit too old-fashioned, and 2) might seem odd when making a plot that is to be placed on a sheet of paper together with other elements like text. size is another name I considered, but as the function already has a parameter called limitsize, I think they would be an odd pair. Anyways, preset is just one suggestion and I can rename it as wished.)

For now, some selected paper sizes and screen resolutions are included, but I'm happy to make the list more comprehensive if that seems reasonable.

Example Usage

library(ggplot2)
p <- ggplot(data.frame(x = rnorm(100), y = rnorm(100)), aes(x, y)) + geom_point()

ggsave("letter.pdf", p, preset = "letter")
ggsave("a4r.pdf", p, preset = "a4r")
ggsave("hd.png", p, preset = "HD")
ggsave("1920x1080.png", p, preset = "1920x1080")