feat(menu): support open an entry in new tab (#294)

* feat(menu): support open an entry in new tab

closes https://github.com/CaiJimmy/hugo-theme-stack/issues/230

* Revert change in markup.tableOfContents.ordered

* Upgrade Netlify Hugo version

* doc: require Hugo Extended >= 0.87.0

layouts/partials/sidebar/left.html

@@ -39,7 +39,7 @@
         {{ $active := or (eq $currentPage.Title .Name) (or ($currentPage.HasMenuCurrent "main" .) ($currentPage.IsMenuCurrent "main" .)) }}
 
         <li {{ if $active }} class='current' {{ end }}>
-            <a href='{{ .URL }}'>
+            <a href='{{ .URL }}' {{ if eq .Params.newTab true }}target="_blank"{{ end }}>
                 {{ if .Pre }}
                     {{ partial "helper/icon" .Pre }}
                 {{ end }}