[ImgBot] Optimize images

*Total -- 315.69kb -> 245.97kb (22.09%)

/.github/github-mark-white.png -- 4.72kb -> 2.83kb (39.98%)
/.github/opencommit-example.png -- 303.97kb -> 236.62kb (22.16%)
/.github/logo-black.png -- 2.11kb -> 1.65kb (21.8%)
/.github/logo-grad.svg -- 3.19kb -> 3.17kb (0.52%)
/.github/logo.svg -- 1.70kb -> 1.69kb (0.34%)

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

Author: ImgBotApp