Menu: Allow to explicitly override if a menu url is remote or not (#809)

* Menu: Allow to explicitly override if a menu url is remote or not

This can be very useful if a hugo website f.e. on GitHub pages is
constructed from different individual spec repos but still represent
one coherent site.

* Rename param to bookNewTab

---------

Co-authored-by: Alex Shpak <alex-shpak@users.noreply.github.com>

layouts/_partials/docs/menu-hugo.html

@@ -17,7 +17,7 @@
     {{- $class = $class | append "active" -}}
   {{- end -}}
   <li>
-    {{- $isRemote := (urls.Parse .URL).IsAbs -}}
+    {{- $isRemote := default (urls.Parse .URL).IsAbs .Params.bookNewTab -}}
     <a href="{{ .URL }}"
       {{- with $class }} class="{{ delimit . " " }}"{{- end -}}
       {{- with .Title }} title="{{ . }}"{{- end -}}