injectStylesheet(doc, section) {
    let style = doc.createElement("link");
    style.setAttribute("type", "text/css");
    style.setAttribute("rel", "stylesheet");
    style.setAttribute("href", this.settings.stylesheet);
    doc.getElementsByTagName("head")[0].appendChild(style);
  }
Hook to handle injecting stylesheet before
a Section is serialized
@param  {document} doc
@param  {Section} section
@private