Allow min/max dates to be set on datepicker input

Allows :max_date and :min_date options to be passed to :datepicker input.

```ruby
f.input :starts_at, as: :datepicker, datepicker_options: { min_date: "2013-10-8", max_date: 3.days.from_now.to_date }
f.input :ends_at, as: :datepicker, datepicker_options: { min_date: 3.days.ago.to_date, max_date: "+1W +5D" }
```