Fallback to `select` if array of `checkboxes` fail

Allows fields that are arrays of strings to be passed to `<select multiple>`
inputs.
I'm against initializers doing work - how about something like:

```
SelectInput.can_fill?(label, value) && SelectInput.fill(label, value)
```

or

```
select_input.new(label, value)
select_input.can_fill_all_options? && select_input.fill
```