EDN extension support

While there is really no significant difference between clj and edn, edn implies to me data file and clj implies executable code, so I would prefer to use .edn extension. 

Its possible to extend the load-config without actually modifying this project, but it would largely be copy and paste since you can't call another multimethod from another that I'm aware and it seems reasonable given that .edn had become more commonplace that the project should support it directly.  

Initially, I just factored out the common method, but derive really makes more sense here to me and does allow others to add support for same behavior different extension using derive rather than copy paste, the only catch is if any other users have added their own defmethod functions using the string instead of the keyword they will need to change to :cardia/extension when bumping version number. Thoughts on that?