Allow overriding `as: :radio_buttons` I18n options

In addition to overriding the default `yes` / `no` and `true` / `false`
options.

In this case, a `collection`-less radio button would do a lookup for
overridden `true` / `false`  I18n values

```yml
simple_form:
  options:
    user:
      active:
        true: "Yes!"
        false: "No!"
```

This would allow for `f.input :active, as: :radio_buttons` to have a radio label
different from the default `Yes` / `No`