Enhancement/Markdown preview code blocks styling

Oni Version: Neovim Version (Linux only): Operating System:

Issue: Having taken to using the markdown previewer more since at @CrossR's updates and finally writing some docs for new features I note some things about the markdown previewer. Code blocks render with a default font which doesn't match any of a user's choices. There is also no padding on the elements so text tightly within the code block. Expected behavior: Code blocks should have font-family: user-text-font-family and have some small padding applied to them.