collapsed menu right arrow, uncollapsed down arrow (#332)

assets/_main.scss

@@ -125,6 +125,14 @@ ul.pagination {
   input.toggle:checked + label + ul {
     display: block;
   }
+
+  input.toggle + label::after {
+    content: "▸";
+  }
+
+  input.toggle:checked + label::after {
+    content: "▾";
+  }
 }
 
 .book-section-flat {

layouts/partials/docs/menu-filetree.html

@@ -34,7 +34,6 @@
       <a {{ if .Page.Content }}href="{{ .Page.Permalink }}"{{ end }} class="{{ if $current }}active{{ end }}">
         {{- partial "docs/title" .Page -}}
       </a>
-      <span>▾</span>
     </label>
   {{ else if .Page.Content }}
     <a href="{{ .Page.Permalink }}" class="{{ if $current }} active{{ end }}">