No account needed. They don't have access to your stuff—it works on local files—but appear to be working on a cloud option. They claim to support Commonmark and

No account needed. They don't have access to your stuff—it works on local files—but appear to be working on a cloud option. They claim to support Commonmark and Github Flavored Markdown. [[filename]] is just a shortcut for [filename](filename), but in any case "Export to standard Markdown links" is on their short-term roadmap. You might also check out Zim.