Feature request: convenience function for saving graphs

[nateGeorge]

I plan on adding this later. A simple function to take care of saving the plot as an image file. Something like:

save.graph <- function(graph, filename, filetype = 'jpg') {
  html.file <- paste0(filename, '.html')
  jpg.file <- paste0(filename, '.', filetype)
  saveWidget(graph, html.file)
  webshot::webshot(html.file, file = jpg.file)
}

Considerations:

• should it auto-load webshot and htmlwidgets?
• should it auto-insall phantomjs if not installed?