fix: browser cleanup (#35)

Cleaning up after the parser when it's done in the browser, before
returning result.

src/mercury.js

@@ -86,6 +86,12 @@ const Mercury = {
       };
     }
 
+    // if this parse is happening in the browser,
+    // clean up any trace from the page.
+    if (cheerio.browser) {
+      cheerio.cleanup();
+    }
+
     return result;
   },