Generate anchors to properties and methods

Hey @redmunds, @peterflynn

This is an initial approach to get named API entries so they can be targeted using anchors (#73). It's not the full solution, but I think it's already quite useful.

I'd like to get this in so you can use it when generating the docs for 0.42. After my vacation, I'll polish the missing details using #79.

What do you guys think?
Seems like `id` is the safer way to go.