GovDelivery doesn't massage api response

It just turns the XML into a struct and makes keys with dashes use underscores
instead, eg `param-id` becomes `param_id`.
Move config into YAML files

Load the config by merging together YAML files in the config directory instead
of having a bunch of constants lying around.

Pass the single config hash into the `Application` on initialization.

Also remove all default parameters from `Application` so that it's used in the
same way in test and production.
Use topic subscription URL from config in service

This link template used to exist in the client but the client has since been
simplified to remove any data massaging.
WIP