[PATCH] ggplot2: fix ggsave('a.svg').

[Ben Woodcroft]

Hi all,

Beforehand, trying to save an SVG file from a plot generated with ggplot did
not work.  'r-svglite' is only a listed as a suggested dependency for gpglot2,
but given that saving SVGs is reasonably central (at least for me), then I
think we should propagate it.

Test code:
$ R
> library(ggplot2)
> qplot(x=c(1,2,3),y=c(3,4,6))
> ggsave('/tmp/a.svg')
Saving 6.99 x 7 in image

Thanks,
ben