fixes #6: add selector for classes of node types

In order for the tests to run properly, we need to wait for Constellation/estraverse#16 and Constellation/estraverse#17. I've taken the liberty of assuming that `1.3.3` will be published after they are merged. If the minor version is bumped instead, I'll update the commit. XML is omitted because it's harder to detect and very uncommonly used.

Fixes #6.