Allow aliasing of profile data on request.

Allows the user to specify, via `options`, the name of the property on `req` that will be set to the decoded profile data. For instance, if I specify:

```javascript
expressJwt({ secret: 'shhhh', alias: 'profile' }) 

// Some later handler.
function(req, res, next) {
    req.user // undefined
    req.profile // user data
}
```