Feat/post start hook

[dreusel]

This introduces the option to configure a post-start-hook for apps. The hook should be implemented in the form of a js file where the main export is the function taking an info object and callback as its parameters:

module.exports = function hook(info, cb) {
  cb(null);
};

This function will be invoked after the app has been (re)started. The info object contains useful things for knowing about / interacting with the newly running app instance:

• {string} info.pid The apps PID
• {Stream} info.stdin The apps STDIN stream
• {Stream} info.stdout The apps STDOUT stream
• {Stream} info.stderr The apps STDERR stream
• {object} pm2_env The apps environment

The hook can be enabled by adding a post_start_hook field to the app configuration blob, for example:

- name: app
  script: app.js
  post_start_hook: post-start-hook.js

Q	A
Bug fix?	no
New feature?	yes
BC breaks?	no
Deprecations?	no
Tests pass?	yes
Fixed tickets	#5705
License	MIT
Doc PR	pm2-hive/pm2-hive.github.io#250

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