Add option for rmForwardContext

This option does not proxy the context of the url to the target server (so `localhost/context/path` gets proxied to `localhost/path`.

Not sure it really warrants a new option (this is more a problem of configuration of the target server) but here's a pull request anyway. What it be more useful to have a callback intercept for url modification? i.e.

```
{
    context: '/something',
    host: 'localhost',
    modifyUrl: function (url) {
        return url.replace("some", "every");
    }
}
```