Allow disabling anchored headings

Usage =>
For site-wide:
    Params.disableAnchoredHeadings: true

For front-matter:
    disableAnchoredHeadings: true

layouts/_default/single.html

@@ -26,7 +26,9 @@
   {{- partial "toc.html" . }}
   {{- end }}
   <div class="post-content">
+    {{- if not (.Param "disableAnchoredHeadings") }}
     {{- partial "anchored_headings.html" .Content -}}
+    {{- else }}{{ .Content }}{{ end }}
   </div>
   <footer class="post-footer">
     {{- if .Params.tags }}