resolves asciidoctor#63 allow layout to be disabled

- false disables layout, enables standalone output
- nil disables layout, does not enable standalone output
- not specified or empty string uses default (inherited) layout

lib/jekyll-asciidoc.rb

@@ -15,6 +15,7 @@ class AsciiDocConverter < Converter
         env=site env-site site-gen=jekyll site-gen-jekyll builder=jekyll builder-jekyll jekyll-version=#{Jekyll::VERSION}
       )
       HEADER_BOUNDARY_RE = /(?<=\p{Graph})\n\n/
+      STANDALONE_HEADER = %([%standalone]\n)
 
       safe true
 
@@ -67,7 +68,7 @@ def setup
           raise FatalException.new("Invalid AsciiDoc processor: #{@config['asciidoc']}")
         end
       end
-      
+
       def matches(ext)
         ext =~ @config['asciidoc_ext_re']
       end
@@ -79,9 +80,12 @@ def output_ext(ext)
       def convert(content)
         return content if content.empty?
         setup
+        if (standalone = content.start_with?(STANDALONE_HEADER))
+          content = content[STANDALONE_HEADER.length..-1]
+        end
         case @config['asciidoc']
         when 'asciidoctor'
-          Asciidoctor.convert(content, @config['asciidoctor'])
+          Asciidoctor.convert(content, @config['asciidoctor'].merge(header_footer: standalone))
         else
           warn 'Unknown AsciiDoc converter. Passing through unparsed content.'
           content
@@ -112,6 +116,8 @@ def render_with_liquid?
         end
       end
 
+      STANDALONE_HEADER = Converters::AsciiDocConverter::STANDALONE_HEADER
+
       def generate(site)
         asciidoc_converter = JEKYLL_MIN_VERSION_3 ?
             site.find_converter_instance(Jekyll::Converters::AsciiDocConverter) :
@@ -135,7 +141,15 @@ def generate(site)
               page.data.update(SafeYAML.load(adoc_front_matter * "\n"))
             end
 
-            page.data['layout'] = 'default' unless page.data.key? 'layout'
+            case page.data['layout']
+            when nil
+              page.data['layout'] = 'default' unless page.data.key? 'layout'
+            when ''
+              page.data['layout'] = 'default'
+            when false
+              page.data.delete('layout')
+              page.content = STANDALONE_HEADER + page.content
+            end
 
             page.extend NoLiquid unless page.data['liquid']
           end
@@ -155,7 +169,15 @@ def generate(site)
               post.data.update(SafeYAML.load(adoc_front_matter * "\n"))
             end
 
-            post.data['layout'] = 'post' unless post.data.key? 'layout'
+            case post.data['layout']
+            when nil
+              post.data['layout'] = 'default' unless post.data.key? 'layout'
+            when ''
+              post.data['layout'] = 'default'
+            when false
+              post.data.delete('layout')
+              post.content = STANDALONE_HEADER + post.content
+            end
 
             post.extend NoLiquid unless post.data['liquid']
           end