src/language-markdown/printer-markdown.js/printUrl
function printUrl(url, dangerousCharOrChars) {
  const dangerousChars = [" "].concat(dangerousCharOrChars || []);
  return new RegExp(dangerousChars.map(x => `\\${x}`).join("|")).test(url)
    ? `<${url}>`
    : url;
}