refactor gift singleton to handle commands better

[Drathal]

closes #79

• refactor gift singleton
• add add_game_command same interface as add_command
• add remove_game_commands
• the new commands handle registering game commands and unregister game commands in selection.gd
• !start handled by gift singleton, emits streamer_start event
• !wait handled by gift singleton, emits streamer_wait event
• fixes bug when calling !start with 0 arguments
• pool_royale refactored to use new system
• cannon refactored to use new system
• numbermind refactored to use new system
• marble_race is so unfinished, nothing changed
• all new games CAN use the new system but don't have to