Add support for `proxyhost` flag

Hi,

I just added support for a `proxyhost` flag as sometimes it could be useful to bind to specific interfaces (e.g. `127.0.0.1`).

I added this flag in order to prevent breaking current proxy's defaults, but maybe something like a `--listen` flag (setting both interface and port) will suits to you better.

Please let me know what do you think.

Regards.