rework bin/generate_docs

Hoping to work on https://github.com/batmanjs/batmanjs.github.io/issues/27, so I refactored bin/generate_docs a bit.  I ran bin/generate_docs and got nearly identical output (you can see the only diff is updated content), so ... it works!

I created a DocGenerator class which gets populated with DocSections. It's not pretty but I figured I'd PR at this stage before I kept going. @pushrax, @angelini does this seem Ok to you guys? If so, I'll forge onward!