Add a game visualizer using the PTN-Ninja API

[ViliamVadocz]

• New --visualize flag that will enable visualization
• Each game gets a separate window and can run concurrently
• The moves are played live using PTN-Ninja's embed API

The code is quite hacky, and I also pull in 3 dependencies, so no worries if you do not want to merge.

How it works

1. The main thread starts a WebSocket server which awaits WebSocket connections. This thread also has a Receiver that will be used to receive a per-game Receiver<visualizer::Message>.
2. Each game thread opens a browser running visualizer.html which contains a PTN-Ninja embed.
3. Once PTN-Ninja is loaded, the window creates a WebSocket connection to the server and is ready to receive PTN-Ninja API messages that it forwards to the embed.
4. The WebSocket connection on the server triggers a creation of a new thread (let's call it a sub-thread) which soon receives a Receiver<visualizer::Message> from a game thread.
5. The game thread talks to to this sub-thread over this channel that was sent over, and it sends a Message::Start, and afterwards Message::Ply for the moves in the game.

Telephone game

PTN-Ninja iframe
🠕 contentWindow.postMessage()
visualizer.html
🠕 WebSockets (JSON)
WebSocket Server sub-thread
🠕 Receiver<visualizer::Message>
game thread