$f3->build: token indexing harmonization with $f3->run

Follow-up of https://github.com/bcosca/fatfree-core/pull/5.

In `$f3->run`, tokens get a numeric index, so `$f3->build` should probably follow the same logic:

```php
$f3->build('/resize/@format/*/sep/*',array(
  'format'=>'200x200',// 1=>'200x200' also works
  2=>'foo/bar',
  3=>'baz.gif'
));
```