What about using your code editor's markdown plugin and real time preview in a browser?
For example with Vim, one of the plugins launches a browser and then you get a real time preview while you type in Vim. The nice thing is you can tell it to use a specific CSS file so you can get a 100% copy of GitHub's README styles. It even syncs your cursor if you want that behavior.
I demo'd the above set up here: https://nickjanetakis.com/blog/writing-and-previewing-markdo...