Double separators for chapter anchors

examples/full.chapters.json

@@ -11,7 +11,7 @@
     "title": "Chapter 2",
     "chapters": [
       {
-        "slug": "2-nested",
+        "slug": "2--nested",
         "title": "Chapter 2.1",
         "chapters": [
 

examples/full.html

@@ -63,19 +63,19 @@ The Rty
 </div>
 </div>
 </div>
-<h2 id="chapter-1-юникод">Chapter 1</h2>
+<h2 id="chapter--1-юникод">Chapter 1</h2>
 <p>
 <span>
 Chapter 1 paragraph text
 </span>
 </p>
-<h2 id="chapter-2">Chapter 2</h2>
+<h2 id="chapter--2">Chapter 2</h2>
 <p>
 <span>
 Chapter 2 paragraph text
 </span>
 </p>
-<h3 id="chapter-2-nested">Chapter 2.1</h3>
+<h3 id="chapter--2--nested">Chapter 2.1</h3>
 <p>
 <span>
 Chapter 2.1 paragraph text

lib/repubmark/elems/chapter.rb

@@ -66,11 +66,11 @@ module Repubmark
 
       def anchor
         if parent.instance_of?(self.class) && @level > 2
-          "#{parent.anchor}-#@slug"
+          "#{parent.anchor}--#@slug"
         elsif anchor_prefix.empty?
           @slug
         else
-          "#{anchor_prefix}-#@slug"
+          "#{anchor_prefix}--#@slug"
         end.freeze
       end