Add paddles

- Create paddle class.
- Add two paddles to game.
- Update game::event_loop to handle input for moving the paddles.
Constrain paddle movement to boundary

- Add paddle collision rectangle.
- Add checks to paddle::update to keep the paddles within the game's
  boundary.
Add documentation

Add documentation to the paddle class.

Closes #3.
Merge branch 'feature/draw-paddles' into develop