ggsave does not handle emf file type

Thank you for creating ggplot.  It looks really good.  I noticed that the ggsave function does not handle the emf file type, just the wmf.  Can I suggest this addition to the save.r script at about line 49 of the current version.

  emf <- function(..., width, height)
    grDevices::win.metafile(..., width=width, height=height)

Many thanks

Charles
