[enhance] facebook: allow the use of a dispatcher

The Facebook token retriever function requires a raw unparsed query
string that it parses immediately. This prevents users from using a
dispatcher because the query string has been parsed already.

This change allows either a query string or a list of tuples to be used
when handling the answer coming from Facebook redirection.

Example:

```javascript
    function connect(query) {
        match (FBA.get_token(query, redirect)) { … }
    }
…
    match (url) {
        case {path:["connect"] ...} : connect(url.query)
    }
```