Added ability to set custom proxy host 

I really like octolog :+1: 

But i had troubles with the automatic resolved host-name running behind another proxy. 

If you have a proxy __http://proxy.example.com__ which redirects to the octolog-proxy __http://localhost:8000__ the redirect urls are always pointing to the localhost url. So i added the option to define the proxyhost as part of the config.json which overwrites the req.headers.host if present. 
This works perfectly fine for me, now :) 


```
{
    "proxyhost" : "proxy.example.com" 
}
```