SnipperApp 2 provides you a full-fledged editor. It supports more than 120 programming languages, and a Github Flavored Markdown.
shift + cmd + L- show & hide line numbers
cmd+ & cmd-- increase & decrease font size
cmd + L- open "Go to line" window
Github Flavored Markdown
SnipperApp 2 supports indented and fenced code blocks when writing markdown.
The same applies to the markdown preview.
Beautiful markdown cheatsheet can be found here.