Support unix sockets in proxy-table.

The routing-proxy provides support for unix sockets via the socketPath option.
This implements this feature for the proxy-table as well. This is done by
interpreting an imaginary socket:// protocol. This enables router
configurations like the following:
```js
    {
      router: {
        'localhost': 'socket:///path/to/socket',
        ...
      }
    }
```