Launch external processes along with the Aino

Example:

watchers = [
  [
    command: "node_modules/yarn/bin/yarn",
    args: ["build:js:watch"],
    directory: "assets/"
  ],
  [
    command: "node_modules/yarn/bin/yarn",
    args: ["build:css:watch"],
    directory: "assets/"
  ]
]

children = [
  {Aino.Watcher, name: MyApp.Watcher, watchers: watchers}
]