[Feature Request] multiple callbacks

[TheCycoONE]

I was attracted to this tool by the callback functionality (#110) but for our purposes (simulating the Twilio Programmable Messaging API) it would be useful to support multiple callbacks for the same request.

In Twilio's API they send you several status updates as the message progresses.

I was thinking:

{
  "request": { ... },
  "response": { ... },
  "callbacks": [ { <callback definition 1> }, { <callback definition 2>} ...]
}