Added the preserve-host-header option when running in proxy mode

As discussed in the Google group, I'm adding the --preserve-host-header option to tell Wiremock that we want it passes the Host header value as it comes from the client when running in proxy mode. If the --preserve-host-header option is not specified, it takes the host name from the url specified in the --proxy-all option.

Because I don't know the code deeply, this might not be the perfect solution. If you have any suggested change don't hesitate in letting me know.

Regarding the reason why I started to use the class WiremockConfiguration as an static holder of the configuration values is because the following reasons:

1. CommanLineOptions makes me thing about it simply as a way to communicate the user's needs to the application, with no logic at all, and to be used only in the highest layer of the application, because it's contact point to the external world.
2. We need to access to configuration values from the lower layers of the application, and I don't thing you want to change the signatures all the way down to pass the values through, mainly because we should keep the app flexible enough to support future changes in the parameter list (it wouldn't be nice to change the signatures again if a new option is added).