[ImgBot] Optimize images (#50)

*Total -- 155.01kb -> 139.51kb (10%)

/src/icon128.png -- 4.44kb -> 2.81kb (36.74%)
/assets/icon-128x.png -- 4.44kb -> 2.81kb (36.74%)
/assets/icon.png -- 22.36kb -> 16.60kb (25.74%)
/src/icon48.png -- 1.90kb -> 1.53kb (19.71%)
/assets/screenshot.png -- 121.87kb -> 115.77kb (5.01%)

Author: imgbot[bot]