Accept ":" in define, as well as "=", and document that in the macro

Part of #80 (closed)

I have not yet gone through all the tests etc., but this will make it possible to start changing them.

@nickm, when do you think we should merge this? The implication is a lot of changes to the docs etc. including in the tutorial. I'm happy to do the actual mechanical editing, since I know you don't really like this change, but I want to do it in a way that doesn't interfere with your docs work.

FTAOD, this means I am happy to wait.