Make export functions also work on systems without a graphics device in non-interactive mode?

[tomwenseleers]

Right now the export functions work on systems in non-interactive mode, provided that they have access to a graphics device (and ggplot2 plots can also be passed as an object). Some working in non-interactive mode, however, do not even have access to a graphics device. Would it be worth to also support that usage case? Feel free to give your thoughts & provide some reproducible example of what you would like to see supported & details on setup (@pearsonca)...