Add html5lib as a submodule
fixup! Add html5lib as a submodule
fixup! fixup! Add html5lib as a submodule
fixup! fixup! Add html5lib as a submodule

Also add six as a submodule.
Merge pull request #1445 from w3c/jgraham/html5lib_submodule

Add html5lib as a submodule