addCodeBlock(code, lang) {
        const attrs = Object.assign({}, (lang && { lang }));
        const element = this.wrap('pre', this.wrap('code', code), attrs);
        return this.addRaw(element).addEOL();
    }
Adds an HTML codeblock to the summary buffer

@param {string} code content to render within fenced code block
@param {string} lang (optional) language to syntax highlight code

@returns {Summary} summary instance