Ignore keyboard event if modifier key is held

so that it does not interfere with web browser's own keyboard shortcut,
e.g. "Alt + Left arrow" for going back to the previous page in
browsing history.

layouts/partials/previous-next-links.html

@@ -13,13 +13,13 @@
 
 <script>
   document.body.onkeyup = function(e){
-
-    {{ if .Prev }}
+    if (!(e.ctrlKey || e.shiftKey || e.altKey || e.metaKey)) {
+    {{- if .Prev }}
       if (e.keyCode == '37') { window.location = '{{ .Prev.Permalink }}'; }
-    {{ end }}
-    {{ if .Next }}
-      if (e.keyCode == '39') { window.location = '{{.Next.Permalink }}'; }
-    {{ end }}
+    {{- end }}
+    {{- if .Next }}
+      if (e.keyCode == '39') { window.location = '{{ .Next.Permalink }}'; }
+    {{- end }}
+    }
   };
-
 </script>