Make ability to traverse a tree by post-order method

[KSXGitHub]

traverseTree and traverseTreeSync use pre-order by default, which means that onDirectory would be called with parent directory before the children.

Sometimes, it needs to be post-order which let the callbacks above executes with the children before the parent. (e.g. deleting tree).

So I suggest either:

• Create traverseTreePostOrder and its corresponding sync version
• Add one more parameter to these functions