Allow datepicker options to be passed to datepicker_input

Allows :max_date and :min_date options (or other jQueryUI datepicker
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"}
```