Parsing of date(time)s as delivered by Rails' form helpers.

Proof of concept, not finished yet. Wanted to start a conversation if this is something you'd consider merging in. Rails' form helpers generate a bunch of `<select>`s for the components of a date(time). These are then sent as "field(1i)", "field(2i)" etc. ActiveRecord is smart enough to convert these upon calls to `new`, `update_attributes` etc.

Given that ActiveInteraction mimics ActiveRecord, it would be nice if `Interaction.run(params)` would be able to process these fields. This however requires some changes since as-is a filter only receives a single value that is already extracted from the original given hash. This is the `all_inputs` parameter I've added throughout. I don't really like what this does to the code though.

If you're interested in having compatibility with Rails' date params, I'd propose that I add a `extract` method to `Filter`:

```ruby
class Filter
  def extract(inputs)
    inputs[name]
  end

  def clean(inputs)
    value = cast(extract(inputs))
    if value.nil?
      default
    else
      value
    end
  end
end
```