Filters

#### A new way of writing filters

Now, the filters are defined in a model.

For example:
```ruby
class ProductsGrid < MightyGrid::Base
  scope { Product }

  filter :name
  filter :company, :enum, attribute: :name, model: Company, collection: Company.all.map { |c| [c.id, c.name] }
  filter :is_active, :boolean
end
```