nav with left/right, added poems

nav.js

@@ -5,6 +5,16 @@
 
   bck.addEventListener('click', function() { idx--; show(); });
   fwd.addEventListener('click', function() { idx++; show(); });
+  document.addEventListener('keydown', function(e) {
+    switch (e.which) {
+      case 37:
+        idx--;
+        break;
+      case 39:
+        idx++;
+    }
+    show();
+  });
 
   function show() {
     var poems = document.getElementsByClassName("poem");