Add support for validating Hashdeep (md5deep, etc.)

If this pull (really, the changes of [this commit](https://github.com/ajnelson/dfxml_schema/commit/086c632abe4906c7ca91a0cfdbb37f141c3bf7d6)) is merged, and if [this pull](https://github.com/jessek/hashdeep/pull/302) is merged, `hashdeep` will produce valid DFXML.