#47 Add .png export

[A. Kruppa]

The 0.21.1 version allowed exporting image files in PNG format, but the 2.0 version allows only JPEG. PNG seems a much more natural choice for diagrams: smaller file than JPEG (at least in my example) and no ugly compression artefacts. Could PNG please be added as an export format again?

Thanks, and thank you very much for Violet.