Traverse generic nodes

Is there a sense to add generic nodes to traversing, such as _Expression_, _Function_, _Node_, _Statement_, _Declaration_, _Pattern_?

That is done in [ast-types](https://github.com/benjamn/ast-types#ast-traversal), and that feature seems to be quite useful.

For example:

``` js
visit(ast, {
  //walk into any node
  Node: function(node){
  }
});
```

Just an idea.
