add filter function to tree data structure

[brucou]

Given a tree, return a tree of the same structure made up of the objects returned by the callback which is executed at each node. If, however, at a given node the callback returns a falsy value, then the current node and all of its descendents will be pruned from the output tree.

Similar to Array.filter

Example : node.filter(function (node, parent_node, index, root_node) {/*SOMETHING*/})

NOTE : this is closely linked to this existing issue : #22

[joaonuno]

Please see my comment on issue #30

I already have a solution to the issue #22 that hopefully will cover the different traversal strategies but have not implemented it yet.

[brucou]

With a reducing traversal, stop at any level is relatively easy to implement : https://github.com/brucou/functional-rose-tree

A function pruneWhen can additionally allow to remove part of the tree, based on a predicate. This is another indirect way to implement stopping conditions (if a part of the tree is removed, it can be traversed).