Add boolean_label_class config

This config is to make possible to change the class of the label that wrap the input in a boolean input.

Before this change we always use `'checkbox'`. Now the default is still `'checkbox'` but it can set to `nil`.

### Before

```ruby
f.input :active, as: :boolean
```

Would generate something like this:

```html
<label class='checkbox'>
  <input type='checkbox'> Active
</label>
```

### After

```ruby
config.boolean_label_class = nil
```

```ruby
f.input :active, as: :boolean
```

Would generate something like this:

```html
<label>
  <input type='checkbox'> Active
</label>
```