Made build method twig context aware

It's usefull to provide a mechanism of using context twig variables. For instance, taking movie names:
Main / Movies / Saw 5
in breadcrumbs.yml:
```yml
Movies:
    label: Movies
    route: movies
    children:
        Movie view:
            label: movie.name
            route: movie_view
```
Then user may do some twig magic to provide twig variable variable resolving and voila.

Another way is to provide getter, wich will return only built breadcrumbs array without rendering template.