improve renderer option handling

Don't waste time constructing a renderer then override it if one is
passed into the options object.

plugin/markdown/plugin.js

@@ -424,6 +424,9 @@ const Plugin = () => {
 
 			deck = reveal;
 
+			let { renderer, ...markedOptions } = deck.getConfig().markdown;
+
+			if (!renderer) {
 				let renderer = new marked.Renderer();
 
 				renderer.code = ( code, language ) => {
@@ -448,10 +451,11 @@ const Plugin = () => {
 
 					return `<pre><code ${lineNumbers} class="${language}">${code}</code></pre>`;
 				};
+			}
 
 			marked.setOptions( {
 				renderer,
-				...deck.getConfig().markdown
+				...markedOptions
 			} );
 
 			return processSlides( deck.getRevealElement() ).then( convertSlides );