Support additional request parameters

I have to support the `prompt` parameter as defined in [OpenID Connect](http://openid.net/specs/openid-connect-core-1_0.html#AuthRequest).

What is the preferred way to support additional parameters in the authorization call to get accepted als PR?

I thought about passing `req.query` to `issue()` (e.g. in `token.js`):

``` js
var arity = issue.length;
if (arity == 4) {
issue(txn.client, txn.user, txn.res, issued);
} else if (arity == 3) {
issue(txn.client, txn.user, issued);
} else { // arity == 2
issue(txn.req.query, issued);
}
```

Maybe we can also use a more generic object for the parameter:

``` js
var obj = {
client: txn.client,
user: txn.user,
res: txn.res,
parameters: txn.req.query
};
issue(obj, issued);
```

Start with token.js and trace how the authorization call invokes the issue() callback and handles request parameters. Compare the existing callback arities with the proposed parameters object; done requires a settled, backward-compatible way to pass prompt and other authorization parameters.

Written by the indexing model from the issue text.