[Whole Web, Node.js PM2] Loggin with PM2

Add config for app's log and error log for PM2.

{
  "apps": [{
    "name": "App1",
    "script": "app1/server.js",
    "log_file": "log/app1.log",
    "error_file": "log/app1-err.log"
  },{
    "name": "App2",
    "script": "app2/server.js",
    "log_file": "log/app2.log",
    "error_file": "log/app2-err.log"
  }]
}

If on Linux & Mac, you might want to enable 'node' user's permission for writing logs.