Add some pre- and post-rendering hooks

This is a monkey-patch approach, but it's up for debate. Thoughts?

Plugins would implement the hooks by overwriting these empty methods.

The other approach is something like
`Jekyll::Filters.add_post_filter(SomeClass)` and that class would have the one
method required by the method and it would be called in level of priority.

/cc @imathis @benbalter @mattr-