Use variables instead of layout Front Matter

_includes/directory-tree.html

@@ -7,7 +7,7 @@
               If the path ends with a slash, the directory itself will be excluded from the tree.
               Default: The root directory.
 
-Parameters that can be passed as include or Front Matter directory-tree hash:
+Parameters that can be passed as include hash, Front Matter directory-tree hash or Liquid variables with directory-tree- prefix:
 
   li          The generated tree is a list item. Do not wrap an <ul> tag around it.
 
@@ -17,11 +17,8 @@
   strong      Wrap a <strong> tag around titles of the pages that have `strong: true` in the Front Matter.
               Default: false.
 {%- endcomment -%}
-layout=<pre>{{ include | inspect }}</pre>
-layout=<pre>{{ page | inspect }}</pre>
-layout=<pre>{{ layout | inspect }}</pre>
-{%- unless include.li or page.directory-tree.li or layout.directory-tree.li %}<ul>{%- endunless %}
-  {%- assign dir_class = include.dir_class  | default: page.directory-tree.dir_class | default: layout.directory-tree.dir_class -%}
+{%- unless include.li or page.directory-tree-li or directory-tree_li %}<ul>{%- endunless %}
+  {%- assign dir_class = include.dir_class | default: directory-tree-dir_class | default: page.directory-tree.dir_class -%}
   {%- if dir_class -%}
     {%- assign dir_li = '<li class="' | append:  dir_class | append: '">' -%}
   {%- else -%}
@@ -94,7 +91,7 @@
       {%- assign prev_page_dir = pg.dir -%}
     {%- endunless %}
     {%- unless pg.name == "index.md" or pg.name == "index.html" -%}
-      {%- if pg.strong and include.strong or page.directory-tree.strong or layout.directory-tree.strong -%}
+      {%- if pg.strong and include.strong or page.directory-tree.strong or directory-tree-strong -%}
         {%- assign strong = "<strong>" -%}
         {%- assign strong_closing = "</strong>" -%}
       {%- else -%}
@@ -113,4 +110,4 @@
     </ul>
   </li>
   {%- endfor %}
-{% unless include.li or page.directory-tree.li  or layout.directory-tree.li -%}</ul>{%- endunless -%}
+{% unless include.li or page.directory-tree.li  or directory-tree-li -%}</ul>{%- endunless -%}

_layouts/doc.html

@@ -1,12 +1,10 @@
----
-directory-tree:
-  li: true
-  strong: true
-  dir_class: "r-docs-subnav list-group-item"
----
 <!-- remove the white space after footer-->
 {% include top.html %}
 
+{%- assign directory-tree-li = true -%}
+{%- assign directory-tree-strong = true -%}
+{%- assign directory-tree-dir_class = "r-docs-subnav list-group-item" -%}
+
 <style>
 	.r-docs-nav {
 		width: 300px;