decodeMarkupSystem(markupSystem) {
    let system = "none";
    if (markupSystem === "markdown" || markupSystem === "md") {
      system = "markdown";
    } else if (markupSystem === true || markupSystem === "html") {
      system = "html";
    }
    return system;
  }
normalize the markup system
@param {boolean|'md'|'markdown'|'html'} markupSystem
@returns {string}