Create a game class to connect player with a game

Is your feature related to a problem? Please describe.

To keep track of goals a player has for a given story, the Game class is created. It connects a story with a player essentially with a many to many relation between players and stories. With this connection, a game has a list of goals for the player to achieve.

Describe the solution you'd like.

The image below describes the proposed solution. The goal class is added later so the goals attribute can be added later. The begin() method should retrieve the opening passage of the game. The go(link: Link) should retrieve the passage connected to the given link.