Allow service-specific bind_addresses

[vulpine]

While one can specify bind_address in the haproxy section of the config, this applies to all frontends globally. I would like to be able to override this per frontend, by specifying it with the port in the haproxy section of my service thus:

      "haproxy": {
        "bind_address": "1.2.3.4",
        "port": "80",
        ....

[jolynch]

Got merged with the Yelp fork over in #130

[jolynch]

@vulpine Whoops I was mistaken to close this. I like this, can you rebase and add a test?

[hashbrowncipher]

I believe this happened in #156.

[jolynch]

Indeed it did