Serves Up should allow for third party plugins

[sdstolworthy]

Describe the solution you'd like
Serves Up should allow for third party plugins. The proposal is to allow plugins to be included in the JSON file or route command. These plugins should expose a common public interface. The current proposal is to allow for a onRequest handler that will intercept a request before a response is sent.

[sdstolworthy]

Note, plugin interface changed to requestInterceptor and responseInterceptor. These are both exported methods in the defined plugin. They should implement the standard Express middleware signature.