adds an object implementing http.ServeMux to allow for use with alternate listeners(TLS, Onion, I2P, etc)

[eyedeekay]

This allows cerca to be used as a "library" of sorts which a user embeds in their own system and deploys with an alternate set of assets. That way, people who want to can set up a listener which uses different forms of cryptography with different properties to protect the content that they are viewing, and easily set up .onion and .i2p versions of cerca forums.