Microsoft Windows and GitHub Flavored Markdown

This major release brings native Microsoft Windows port of MindForger and support of GitHub Flavored Markdown (via replacement of Discount with mark-gfm - GitHub's fork of cmark, a CommonMark parsing and rendering library).

Big thanks to bonitoo.io and Ivan Kudibal who decided to contribute to MindForger - kudos to Vlasta Šaman Hájek who ported MindForger to Microsoft Windows in one month!