Add support for censorship detection in HTTP Host based on Domclass

Domclass allows us to automatically detect if a site is censored or not based on it's contents DOM structure eigenvalues.

We need to add support for DOM class based automatic censorship detection in HTTP Host.

It would be nice if the person running the test could also select another classifier that is not domclass and easily swap it out.

Other ideas for possible classifiers are detailed in: https://trac.torproject.org/projects/tor/ticket/6180.