Edit Join and Leave Messages

Allow this plugin to edit join and leave messages.
- Include allowing Placeholder API placeholders.
- Include allowing MiniMessage parsing.

A cool use case, albeit not necessary, would be something like...
```
<bold><gray>[<red>-</red>]</gray><bold> <yellow>PLAYER has left the game.</yellow>
<dark_gray><hover:show_text:"%disconnect_reason%">[?]</hover></dark_gray>
```