Add transformers/node visitors

[wouterj]

It would be great if there is a supported way to traverse the generated DocumentNode and modify the node structure.

The docutils.transforms namespace includes quite some usage of such node visitor, some directly applicable to this library:

• Using PendingNodes to better handle the .. class:: directive (see discussion in #141 (comment) )
• Allow auto-numbering sections (ref)
• Generating table of contents from a document node (ref)