Add optional NestingUniqueHeadCounter header ID generator

* Move nesting generator to separate class

* Major bug fix, code simplification

* Get rid of global value, use class variable instead.

lib/nesting_unique_head.rb

@@ -0,0 +1,22 @@
+# Nested unique header generation
+require 'middleman-core/renderers/redcarpet'
+
+class NestingUniqueHeadCounter < Middleman::Renderers::MiddlemanRedcarpetHTML
+  def initialize
+    super
+    @@headers_history = {} if !defined?(@@headers_history)
+  end
+
+  def header(text, header_level)
+    friendly_text = text.parameterize
+    @@headers_history[header_level] = text.parameterize
+
+    if header_level > 1
+      for i in (header_level - 1).downto(1)
+        friendly_text.prepend("#{@@headers_history[i]}-") if @@headers_history.key?(i)
+      end
+    end
+
+    return "<h#{header_level} id='#{friendly_text}'>#{text}</h#{header_level}>"
+  end
+end

lib/unique_head.rb

@@ -14,4 +14,4 @@ def header(text, header_level)
     end
     return "<h#{header_level} id='#{friendly_text}'>#{text}</h#{header_level}>"
   end
-end
+end