Issue title: Silently restart with additional child processes
 Issue body: I'd like to add a vote for this TODO entry: 

> When watching, it'd be good to perhaps bring up a new child and then kill the old one gently, rather than just crashing the child abruptly

 Comments: 
 Comment 0: I think this would probably need to be off by default and enabled with a flag, because if the child is using a resource (for example, if it's a server bound to a port), starting a new process will fail with the resource already in use.

 Comment 1: Unless there's a way to bring up a new child process on the same port and they would share. Basically Supervisor would become the proxy to the child processes.

 Comment 2: Unfortunately, no. Supervisor is (by design) agnostic about what the child
process is doing, so it can't dip its fingers into ports and whatnot.

 Comment 3: Closing, as I think this is unlikely to happen, sorry. It's more of a production use-case, and supervisor is more of a development tool.

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