Support host binds other than 0.0.0.0

I used `elm-reactor` in a coffee shop on a public WiFi a while back and noticed some traffic on my machine. Someone or something appeared to have been running vulnerability scanner against the open port.

Hence, I thought it might be useful to restrict the host that is bound to to `127.0.0.1` for these cases.
It's okay :) In general, narrow focus is better, but I think this is something that'd never change otherwise.