Inline syntax for variants

In most cases, when setting variant specific code, you're not sharing any code within format.

Inline syntax can vastly simplify defining variants in those situations:

```ruby
  respond_to do |format|
    format.js { render "trash" }
    format.html do |variant|
      variant.phone { redirect_to progress_path }
      variant.none  { render "trash" }
    end
  end
```

Becomes:

```ruby
  respond_to do |format|
    format.js         { render "trash" }
    format.html.phone { redirect_to progress_path }
    format.html.none  { render "trash" }
  end
```

:tropical_drink: