_normaliseRouteDefns(routeDefns) {
    if (!Array.isArray(routeDefns))
      routeDefns = [routeDefns];
    routeDefns.forEach((defn) => {
      let fmt = defn.format || "html";
      fmt = fmt.toLowerCase();
      if (fmt === "markdown")
        fmt = "md";
      defn.format = fmt;
    });
  }
Normalise route definition arrays
@param {Array<routeDefinition>} routeDefns Route definitions to normalise