Support a "replyWith" in mcrew services

Currently `cmd/mcrew` has an internal service for HTTP requests. That service supports `"replyTo":MACHINE_ID` in a request, which will cause the response to include `"to":MACHINE_ID`.

As an aid to writing a machine specification that will execute "flows" (more on that later), `"replyWith":SOME_DATA` in a request should result in the response including `"with":SOME_DATA`.