Define Prince version per project

[arthurattwell]

Thank you for this great module. At Electric Book Works, we're beginning to use it to make setup easier for new team members (i.e. no separate Prince installation).

We have a further need: we need to be sure that, on a given project, every team member uses the same version of Prince. In our particular case, our extensive book-design CSS library was created for Prince 11, and isn't entirely compatible with later versions.

I've forked and modified node-prince to meet our needs in electricbookworks#1.

With those modifications, we can now set the Prince version we need in a project's package.json. Our node-prince checks for that before downloading Prince.

We're still testing this approach and may still find issues with it, though it's looking very promising so far.

Would you be interested in implementing this or something similar in your module? I'd be happy to PR my changes, though you might prefer a different approach. My Javascript skills are not as sophisticated as yours! It would certainly be better for us to use your module directly, rather than maintaining our own fork.