Add reference counting to all nodes

[clayrisser]

During my implementation of tables, I found it quite handy to add reference counting support through node arrays.

Instead of reference counting with integers, I am using arrays of nodes.

This solved two major challenges.

1. Knowing where I am in nested nodes of the same type
2. Finding information from nodes outside of the current node

You can see a simple implementation of this with tables.

    tables = []

    def visit_table(self, node):
        self.tables.append(node)

    def depart_table(self, node):
        self.tables.pop()

To detect how deep you are in the tree (at least with nodes of the same type), you can simply grab the length.

    @property
    table_depth():
        return len(self.tables)

To grab the current node of a certain type, simply grab the last node from the array.

    @property
    current_table():
        return self.tables[self.table_depth - 1]

This has been so helpful with tables, I am thinking about adding this to all the nodes. Please give your feedback below.

[clayrisser]

To fully implement reference counting across all nodes, the doctree2md.py would eventually need to be fully deprecated. I think this is reasonable, as I am getting a decent grasp on how sphinx/docutils builders work.

Please referecne the issue below on deprecating doctree2md.py.
#9

[clayrisser]

DISCONTINUED: This project is no longer maintained. The active version of this project is now being maintained at liran-funaro/sphinx-markdown-builder.