Add support for a custom comparator function.

[segevfiner]

Keys are currently compared using <, it would be nice to be able to specify a custom comparison function for cases where you want to use the tree as a set and need to key values in a different way.

It would be also interesting to create some version/optimization for this cases in terms of only having a key without a value.

P.S. Some methods of AvlTree such as getLeft/getRight end up returning BinarySearchTreeNode instead of AvlTreeNode.

[eyas-ranjous]

v5.0.0 released https://github.com/datastructures-js/binary-search-tree#constructor