Create enums relevant for ExportConfiguration

[IndrajeetPatil]

In the following class:

ExportConfiguration$new(
  name = NULL,
  format = NULL,
  width = NULL,
  height = NULL,
  units = NULL,
  dpi = NULL
)

I am assuming the supported file formats are the same ones supported by ggplot2?

one of "eps", "ps", "tex" (pictex), "pdf", "jpeg", "tiff", "png", "bmp", "svg" or "wmf" (windows only)

If yes, I think we can create enum for the same.

Similarly, we can also create an enum for units specifying plot dimensions:

Plot size in units ("in", "cm", "mm", or "px")