ProxyTable updated to support 'default' route

Towards the end of putting this together and testing it works, I found a solution combining regex entries in the proxy table to achieve a default to similar effect. But, this certainly makes for a tidier solution so I'll put it forward.

Have included an example in this pull request to demonstrate what I'm trying to do.

The ProxyTable will now support a `'default'` entry (unless `hostnameOnly:true` is flagged).

```
var options = {
  router: {
    // normal proxy
    'localhost/proxied': 'localhost:9000',
    // default proxy if no match above,
    // this would also work if set as 'localhost'
    // but intended for more complex scenarios
    'default': 'localhost:9001'
  }
};

httpProxy.createServer(options).listen(8000);
```