[ImgBot] Optimize images

*Total -- 179.37kb -> 92.52kb (48.42%)

/readme/highlight.png -- 15.44kb -> 6.00kb (61.15%)
/readme/color-color.png -- 6.82kb -> 2.82kb (58.67%)
/readme/suggest-key.png -- 15.41kb -> 6.73kb (56.32%)
/readme/color-derive.png -- 7.80kb -> 3.67kb (53.03%)
/readme/suggest-common.png -- 10.03kb -> 4.90kb (51.1%)
/readme/color-context.png -- 45.34kb -> 23.24kb (48.74%)
/readme/shape-svg.png -- 23.76kb -> 12.48kb (47.49%)
/readme/color-lineargradient.png -- 9.56kb -> 5.13kb (46.38%)
/readme/font.png -- 26.73kb -> 15.79kb (40.92%)
/readme/suggest-current-id.png -- 5.38kb -> 3.43kb (36.33%)
/readme/key.png -- 13.09kb -> 8.34kb (36.33%)

Signed-off-by: ImgBotApp <ImgBotHelp@gmail.com>

Author: ImgBotApp