change Host header in proxy-all mode.

Hi,

First of all, I like this product very much. It exactly what I need for my current project.
Can you look into the following issue:

If you run the standalone version with the --proxy-all option, the 'Host' request header is forwarded to the destination server. In my case this was localhost:8080. The service could not handle this, and came back with an 'unknown route to host' error. The solution  was for me to change the 'Host' header in my firefox rest client to the same host as the destination server.

It would be a nice feature if it is possible in wiremock to control the Host header that is sends to the destination server.