Markdown-escape HTML tag mentions

Basically, ignoring weird/malicious `class`es, this saves me from having to do GitHub-Flavored Markdown-escaping after the fact on the LMVTFY side of https://github.com/cvrebert/lmvtfy/issues/58.
Oh, how I wish `<label>` rendered as "\<label\>" or "`<label>`" rather than "<label>" (i.e. nothing).