Allow date range to be passed to datepicker_input

Allows :max_date and :min_date options to be passed to :datepicker
input.

```ruby
f.input :starts_at, as: :datepicker, min_date: "2013-10-8", max_date: 3.days.from_now.to_date
f.input :ends_at, as: :datepicker, min_date: 3.days.ago.to_date, max_date: "+1W +5D"
```