Add HashFilter :any option

This adds an `:any` option to `HashFilter` which defaults to `false`. When `true`, any value is permitted. 

I have a number of models in my application which accept arbitrary JSON data from the user and I'd like to remove the need for serializing this data into a string when calling a mutation command. With this change, my commands look like:

``` ruby
class Update < Mutations::Command
   required do
      array :email_templates do
         hash do
            required do
              integer :id
              string :identifier
              string :subject_template
            end

            optional do
              string :from
              string :text_template
              string :html_template
              hash :data, :any => true
            end
          end
      end
   end
end
```