Redirect subdomain to different base path

@tblobaum Just added and option on top of the multiple domain support to allow a subdomian URL to be rewritten to a different base path. Now, if you set the subdomain as:

```javascript
.use('api', { newbase: 'otherbase' }
```

a call to ```api.domain.com/path``` will be rewritten to ```/otherbase/path``` instead of ```/api/path```.

Added tests and updated readme file too.