ggsave: Save a ggplot (or other grid object) with sensible defaults

## Not run:  # ggplot(mtcars, aes(mpg, wt)) + geom_point() #  # ggsave("mtcars.pdf") # ggsave("mtcars.png") #  # ggsave("mtcars.pdf", width = 4, height = 4) # ggsave("mtcars.pdf", width = 20, height = 20, units = "cm") #  # unlink("mtcars.pdf") # unlink("mtcars.png") #  # # specify device when saving to a file with unknown extension # # (for example a server supplied temporary file) # file <- tempfile() # ggsave(file, device = "pdf") # unlink(file) # ## End(Not run)