Add ability to traverse tree in post order

Instead of #512, there could instead be an option to traverse in post order.

For example:

```ts
enum TreeOrder {
Pre = "pre",
Post = "post"
}

node.forEachDescendant(TreeOrder.Post, (node, traversal) => {
console.log(node.getText());
});

node.getDescendantsOfKind(SyntaxKind.ClassDeclaration, TreeOrder.Post);
```

See [this comment](https://github.com/dsherret/ts-simple-ast/issues/512#issuecomment-447601251).

Also, when the tree order is pre and a "node was forgotten" error occurs, it should suggest to the user to use post order.