Process events handling API

[sazarkin]

Can we add a bindings for process.on() ? https://nodejs.org/api/process.html

Something like this:

[@bs.send.pipe: Node.Process.t]
external on :
  (
  [@bs.string]
  [
    | `unhandledRejection(string => unit)
    | `SIGINT(unit => unit)
    | `SIGTERM(unit => unit)
  ]
  ) =>
  Node.Process.t =
  "";