Use cluster to spawn child-processes.

[annitya]

Fixes #10.
Found other pull-request too late :-/

No options required though!

[andreasrs]

I tested this locally and it seems to work well. Thanks @WhiteFire

For those interested, it works because of:
https://nodejs.org/api/cluster.html#cluster_cluster_settings

inspectPort <number> | <function> Sets inspector port of worker. This can be a number, or a function that takes no arguments and returns a number. By default each worker gets its own port, incremented from the master's process.debugPort.

Related issue in child_process and why this was a problem here: nodejs/node#8495 (comment)

I think this is preferable to the approach of making inspect a part of settings explicitly, as suggested in #10
It "just works" without introducing extras.

Thoughts @voldern @dlmr ?

[voldern]

Cool, if this works then its 👍 from me

[dlmr]

Released in 1.0.0-beta.4 🎉