slack-emojinator

npm install -g slack-emojinator

run

slack-emojinator --config config.json --env env-name

config

{
  "env-name": {
    "org": "slack organization subdomain",
    "cookie": "open up the Network tab in DevTools and copy your cookie session here",
    "icons": "/absolute/path/to/icons/location"
  }
}

Name the icon files with their respective shortcodes, for example: trollface.png will become :trollface: in Slack.