Parameterize content types. Closes #3

I think the best way support #3 is to allow `:malformed?` to be overridden and provide the `malformed?` helper function to make it easy to do so. So `malformed?` is now responsible for

 * converting the request body into a string
 * putting the result of parsing the request into `::parsed-output`
 * converting any errors into a string and setting an appropriate :representation

See how `edn-malformed?` is defined for an example of how to customize your application's check for whether a request is malformed.