Attempt to allow min/max dates on datepicker

This isn't ready yet. This doesn't work and I have no idea why.

```ruby
f.input :starts_at, as: :datepicker, min_date: 3.days.ago.to_date, max_date: 3.days.from_now.to_date
```

The field rendered should have the attributes `data-min-date` and `data-max-date`. I have several of my own inputs that work exactly this same way, but this doesn't do it. Am I missing something about formtastic or AA that would interfere?