Add a hook to transform text prior to saving .html

[iabw]

I would like to be able to hook into code-block text before it gets syntax highlighted. This is similar to #59 but technically very different, I think.

I hacked docker.js to replace the module names in my define and require calls with links to the proper file. I find it super useful, but I had to add code in several (seemingly) arbitrary places to make it work.

It's not ready to bring in, but I could get what I wrote on a fork if someone wanted to check it out and try to write something better.