Allow QR library substitution or enable broader variation of output image formats

[msporny]

James Chartrand wrote:

One small change I made was to substitute a different QR library (into a fork of vpqr) so that I could get a PNG image. The PDF library I used wouldn’t take the image type that vpqr normally produces (I can’t remember what that image type was).

Perhaps we can allow QR library substitution or enable a broader variation of output image formats. Clearly, we don't want to do this if it's going to unnecessarily bloat the library.