Add ContextManager pattern to MatchManager and RenderingManager plus default color

[NicEastvillage]

It is now possible to do

# Automatically call ensure_started and disconnect
with MatchManager(RLBOT_SERVER_FOLDER) as man:
    man.start_match(MATCH_CONFIG_PATH)

and

# Automatically call begin_rendering, set_default_color, and end_rendering
with renderer.context(group_id='shot', default_color=renderer.red):
    renderer.draw_line_3d(car.pos, ball.pos)
    renderer.draw_line_3d(car.pos, goal.pos)
    renderer.draw_line_3d(ball.pos, goal.pos)

The addition of default color is also new.