Add ability to document filters

[tfausak]

Similar to #44, I'd love to be able to add programmatically-accessible documentation to individual attributes. This would require adding the ability to inspect an interaction's fitlers programmatically, too. Something like this:

class Interaction < ActiveInteraction::Base
  boolean :a, desc: "It's an attribute, alright!"
  def execute; end
end

Interaction.inputs[:a]
# => "It's an attribute, alright!"

[tfausak]

This is blocked by #52. Until there's a way to get at the attributes, there's no point in adding descriptions to them.

[tfausak]

This is going to look like this now:

class ExampleInteraction < ActiveInteraction::Base
  boolean :a, desc: 'true or false'
end
ExampleInteraction.filters.first.desc
# => "true or false"