Start debugger for running server process

[davewasmer]

Node's debugger mode can be started on an already running process by sending SIGUSER1. Since the CLI runs the server in a separate process, we could make this easier, i.e.:

$ denali server
cli v0.0.14 [linked]  | denali v0.0.32 [linked]

✔ my-app build complete (3.751s)
[2017-09-06T04:19:36.552Z] Server started

Press (d) to start debugger, Ctrl-C to quit

Might be worth exploring a bit further what we could display in CLI output as well. Perhaps we could add Press (f) to fork to test clustering, show CPU/memory usage, etc.