Docutils: Documentation Utilities Code

Attribute validate functions:

* convert string representations to correct data type,
* normalize values,
* raise ValueError for invalid attribute names or values.

The `nodes.Element.validate()` function reports a warning
for validity problems if `self.document.reporter` is available
and raises a ValueError if not.

Testing revealed problems with the "recommonmark_wrapper" parser:

* Validating should be done *after* the "clean up" operations.

* One test case uses an invalid class argument (underscore not allowed
by Docutils). As this sample tests an "only Sphinx" feature,
we just drop it from the Docutils test suite.