API for registering command line commands

[loppear]

Currently it is possible (for e.g. a command to build js assets outside of app startup) for a module to act as a run-and-exit command with the following coordination:

• A config flag to run whatever process and then stop & exit:
  if (this.config.commandName) this.runCommandAfterLaunch.then(::app.stop).then(::process.exit)
• Disable the router module and application file watcher (there's a potentially larger set you might want to disable, but hard to reason about whether any might be contributing to the application behavior you want to command)
• Combine all these flags into an npm script in the application file, e.g.
  DEBUG=nxus:module:* nxus_router__disabled=true nxus_module__command=true npm start

Ideally these patterns could be supported more directly.

[loppear]

Current need for that nxus/clientjs#13
Also probably beneficial for worker-only deploys of the worker-queue processes.