Feature: Conditionally rendering based on a property or function

[zachriggle]

Currently, all elements in the tree are printed with print_tree.

It would be nice if there were support for an optional argument, say filter which is called before displaying any given tree element. You could then do:

def should_print_node(node):
    return node.someProperty != None

pptree.print_tree(tree, filter=should_print_node)

To only print out node which have a given property. This is useful in filtering the tree without having to manually traverse it and remove child nodes in order to get the "filtered" output.