This is the plugin MarkdownLivePreview. I have recently (16 November 2019) rewritten everything so that it is easier for people to contribute.

Hopefully, I'll get around writting a blog post about it soon.

To install it, it's on Package Control

To open the preview, just open a markdown file, and in the command palette, you should find MarkdownLivePreview: Open Preview

If you want to use a keybinding, just add this to your keybinding file

    "keys": ["alt+m"],
    "command": "open_markdown_preview"

FIXME: upload a quick demo gif. (wanna send a pull request?)