Added ability to optionally return node depth

[williamgilpin]

Hello, thanks very much for this fantastic package. For an application I'm working on, I need to be able to get the node depth when I look up a string in the suffix tree. This is usually smaller than the depth property, since it treats multi-character nodes as a single hop as we move down the tree.

I implemented this using optional arguments, to avoid breaking the existing API. It simply consists of adding a counter to the find_path function, which increments whenever node is updated to point to a child of the current node.

It might be possible to instead implement this as an attribute of each node, which is set whenever the tree is first constructed, but I don't know enough about McCreight's Algorithm to implement this myself.