Add a table of contents

Create a new table_of_contents partial that includes an auto-generated table of contents if the word
count is over 400 and the "toc" page variable has been set true.

layouts/_default/term.html

@@ -7,6 +7,8 @@
   <h1>{{ .Title }}</h1>
 </header>
 
+{{ partial "table_of_contents.html" . }}
+
 {{- $hasContent := false -}}
 {{ with .Content }}
   {{- $hasContent = true -}}