Add A3 custom print options to example

[vool]

Hi Rowan,
Just thought it might be good to have an example of customs print sizes in your demo.
There is commit here is you think it's a good idea,
keith

[rowanwins]

Hey @vool

Thanks for the pull request. I've got a bit of leave next week so I'll take a look at this then and get back to you if I have any q's. Got a few things to tidy and release and like the idea of adding a custom demo :)

Cheers
Rowan

[Tomas2D]

Any progress? Thanks for a great library!