feat: allow to extend renderer

Closes #46.

lib/get-marked-with-highlighter.js

@@ -1,23 +1,25 @@
 const marked = require('marked');
 const { getLanguage, highlight } = require('highlight.js');
 
-const renderer = new marked.Renderer();
-
-renderer.code = (code, language) => {
-	// if the given language is not available in highlight.js, fall back to plaintext
-	language = (getLanguage(language) && language) || 'plaintext';
-
-	return `<pre><code class="hljs ${language}">${highlight(language, code).value}</code></pre>`;
-};
-
 /**
  * Get a marked renderer with an attached highlighter.
  *
- * @param {Object} options Marked cnfiguration object
+ * @param {Object} options Marked configuration object
  *
  * @returns a marked renderer with highlight.js parser attached
  */
 module.exports = options => {
+	const renderer = options.renderer || new marked.Renderer();
+
+	if (!renderer.hasOwnProperty('code')) {
+		renderer.code = (code, language) => {
+			// if the given language is not available in highlight.js, fall back to plaintext
+			language = (getLanguage(language) && language) || 'plaintext';
+
+			return `<pre><code class="hljs ${language}">${highlight(language, code).value}</code></pre>`;
+		};
+	}
+
 	marked.setOptions({ ...options, renderer });
 
 	return marked;