Pre-parsing events

[neochief]

I have a site with a huge amount of markdown content that uses inline references extensively. Prior to 0.19 I was able to implement reference map caching by overriding DocParser, but now it's final, so it's no longer possible.

It seems that the ability to pre-fill the reference map was envisioned from the very beginning (#359), so this pull request is a basic attempt to make it happen with events.

[colinodell]

This looks great! And it's a perfect use of the new event system :)

However, I do need to complete #428 first so that we can document new features like this one (and #420) in the proper location. I'll plan to tackle that this week so we can rebase this and target the 1.4 docs instead.

[neochief]

Awesome, thanks!

[colinodell]

Thanks!