Add usage docs & help command

[jkrems]

Minimal requirement is to match what node debug does:

> help
Commands: run (r), cont (c), next (n), step (s), out (o), backtrace (bt), setBreakpoint (sb), clearBreakpoint (cb),
watch, unwatch, watchers, repl, exec, restart, kill, list, scripts, breakOnException, breakpoints, version