Add option to sort the tree

Hi,

In our fork (https://github.com/arr-ai/wbnf/blob/master/gotree/gotree.go) we have added an option to sort the tree, our use-case wants to sort the whole tree based on the leafs which have numbers as the prefix.

Is this something we should look into getting upstream?

[d6o]

Sure! I would love to merge it.

cool, I'll try to get a PR through in the next few days