Rotate around axis relative to camera

[chrisprice]

Fixes #658.

Disclaimers 😄

• I've opted for a relatively lightweight change. I can think of more intrusive changes which extend the types around Transform but didn't want to jump into that without discussion.
• I'm also not an expert in this area, I've just been following through this article. There are likely to be mathematical simplifications that I've missed.
• I've noticed discussion about removing the dependency on nalgebra so I haven't dug into its API looking for appropriate implementations of the logic I've added. There's very likely to be something in there that I've missed.

[hannobraun]

Thank you, @chrisprice, this works great!

Sorry for the delay in reviewing this. Monday was a public holiday here in Germany, and I spent most of it away from the computer.