allow custom prince options

[blowback]

I wanted to experiment with javascript code highlighting engines; this requires the '--javascript' option to be appended to the prince command line.

[revdavethompson]

I have been looking at modifying the pdfkit gem to work with princexml. This would mean that you could pass prince options via html headers, or from within the config.yml file.

We certainly could go the route of passing the options via the command line, but I would need to rework bookshop's code structure a bit more for that.

This also means, however, that I would need to create a princexml gem, which wouldn't be too hard to do.