added :host to options on server start

Hello Zach,

I'm in situation where I need explicitly give a hostname.
I found this https://groups.google.com/d/msg/aleph-lib/HXtY1VnqcEo/ZsSKWjRotX0J. But :host not supported yet  so I have tryed to add it.

P.S: please note that I am a Clojure noob.