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
```