Add the ability to describe input filters 

This is a minor enhancement to the filter configuration DSL that allows for developers to provide a brief human readable description of the filter / argument for the command.

```ruby
class CreateBook < Mutations::Command
  required do
     desc "The Title of the book" 
     string :title
  end
end
```

In large projects which have a lot of commands, the ability to generate API documentation for those commands is very nice and the mutations gem already allows for a great level of introspection and so this is a natural extension of that.

Previously I solved this by adding the 'description' key to the filter options, but I like this approach as inline documentation becomes more of a first class citizen.