In order to run this app:

• Install node.js.
• Clone the repository.
• Install dependencies using npm install.
• Get an API token using these instructions.
• Figure out the identifier of the player you want to control by hitting https://plex.tv/api/resources.xml?auth_token=api-token and grab the clientIdentifier value for the player.

Then run the app as follows:

$ TOKEN=api-token PLAYER=client-identifier-to-control node index.js

Finally, add the webhook to https://app.plex.tv/web/app#!/account/webhooks (it'll be http://localhost:10000).