Honour the 'host' option.

Currently, fb-flo always binds on 0.0.0.0 (the wildcard pseudo IP address) for me, no matter the `host` setting. I could not find any place that uses options.host, so I added the argument to `httpServer.listen`. Looking at the output of `netstat -ltn`, we now listen on the correct host.

The bug seems to have existed since the initial commit in [0516f43](https://github.com/facebook/fb-flo/commit/0516f43ad6225146a69a9bc3370022107232e6eb#diff-c945a46d13b34fcaff544d966cffcabaR21).