Pass debug flags to node

Even if it requires only running one, non-clustered process, allowing `--debug` and `--debug-brk` on `koa-cluster` itself and passing that through to node will allow people to debug without changing their code to make exceptions for running in non-clustered mode.

No file or test is named in the issue. Start by locating the koa-cluster command entry point and the code that launches the Node process; verify that --debug and --debug-brk are accepted and forwarded for the non-clustered case, with tests covering the resulting invocation.

Written by the indexing model from the issue text.