Variants

Inspired by ActionPack Variants. They work as optional segments for
a more fined control of global/specific initializers.

```javascript
page.at('dashboard', function() { /* generic code */ });
page.at('dashboard+user', function() { /* specific code */ });
page.at('dashboard+admin', function() { /* specific code */ });
```

In this example, the page `dashboard+user` will call both `dashboard` and
`dashboard+user` initializers, while `dashboard` will just call the
generic `dashboard` initializer.