support binding to host other than 0.0.0.0 in dev server

[mxmul]

This was discussed in #555, but it seems like only "hostname" was actually implemented (controlling what url the browser opens up by default). I'd like to be able to actually bind the HTTP server to a different interface (e.g. so I could run multiple dev servers bound to :8080 on the same machine).

I'm thinking this could be implemented as a new config option devOptions.host, and would default to 0.0.0.0. Happy to implement this myself if y'all think it makes sense.