[ImgBot] Optimize images (#226)

*Total -- 1,136.20kb -> 834.25kb (26.58%)

/math/images/unit_circle.png -- 26.31kb -> 10.24kb (61.06%)
/red-green-blue-gophers.png -- 673.00kb -> 455.76kb (32.28%)
/epub-cover.png -- 183.40kb -> 153.07kb (16.54%)
/math/images/unit_circle_params.png -- 31.65kb -> 26.57kb (16.07%)
/math/images/unit_circle_12_oclock.png -- 33.99kb -> 28.65kb (15.7%)
/math/images/unit_circle_coords.png -- 28.71kb -> 24.36kb (15.15%)
/red-green-blue-gophers-smaller.png -- 159.13kb -> 135.59kb (14.79%)

Signed-off-by: ImgBotApp <[email protected]>

Author: krynv