Extend outgoing vinyl objects with BEM-related info

Shortly, like that:

``` js
class BemVynil extends Vynil {
  entity: BemEntity,
  tech: String,
  level: String
}
```

This will be useful to build plugins like `streamFilter(file => file.entity.block !== 'button')`.

We can add it as an option that will turn this logic on. Something like `{ bem: true }` or more sensible ;-) 
