CLI - Possible solution to workflow issues

Trying to solve #2 and #4 with a comprehensive CLI.

**TODO**
- [ ] Get list of games
  - Should be a list of branches on all forks for parent repo
- [ ] Join an open game
  - Push access to repo? Probably has to be done manually.
  - Maybe joining a game is creating an issue on the repo with an @mention
- [ ] Start a game with or without an opponent
  - Forks if necessary, creates branch, and opens repo in browser (user has to add opponent as collaborator)
- [ ] Rematches
  - New branch - somehow notify opposing player