Let user specify a hook to configure webapp in the config file.

This allows customization in the web panel without forking, that lets user set up e.g. alternative authentication schemes. I can e.g. configure authentication to just use SSO headers left by frontend Apache, or (sample below) hard-code username for local tests:


```javascript
    configure_server: function (app) {
        app.enable('trust proxy');
        app.use( function(req, res, next) {
            req.session.authed = true;
            req.session.username = 'japhy';
            next();
        });
    },
```