Add any/all support for variants

Same as `format.any`, you can do the same with variants now.

It works for both inline:

```ruby
respond_to do |format|
  format.html.any   { render text: "any"   }
  format.html.phone { render text: "phone" }
end
```

and block syntax:

```ruby
respond_to do |format|
  format.html do |variant|
    variant.any(:tablet, :phablet){ render text: "any" }
    variant.phone { render text: "phone" }
  end
end
```

Regarding implementation, `VariantFilter` was removed and `VariantCollector` is used for both inline and block syntax. This is needed, as now we don't know what variant should be rendered until we collect all of them (`any` complicates this).

In the end, I think this is even better. Implementation is cleaner and more consistent because we don't have 2 different things - filter and collector - but only one, which takes care of everything.