Allow specifying minimum number of workers

[omus]

As of the fix in #69 if workers are not spawned by the pending_timeout then the cluster manager will proceed with the workers currently available. In some scenarios this may be unacceptable so having a way of specifying a minimum number of workers may be convenient.

[ericphanson]

I think maybe it’s ok to just ask (/ document the need for) the caller to check how many pids they get back and handle it there, since maybe the right response is to error or maybe it’s to ask again with less memory or something.