Custom events and API

The game is quite simple and it doesn't persist anything.
Therefore, the game is a great base as minigame and should be open for extension.

This can be done by including custom Spigot events and an API interface:
- `MemoryQueueJoinEvent`
- `MemoryQueueJoinEvent`
- `MemoryCardClickEvent`
- `MemoryEndEvent`
- ...

The API interface should have methods to join and leave the queue.