Added per post excerpt_separator functionality, so you are able to

specify :excerpt_separator (as well as just :excerpt) key direct inside
the post YAML, to make an excerpt based on the value in the post. Tests
were also added.

.gitignore

@@ -12,6 +12,7 @@ gh-pages/
 site/_site/
 coverage
 .ruby-version
+.ruby-gemset
 .sass-cache
 tmp/stackprof-*
 .jekyll-metadata

lib/jekyll/excerpt.rb

@@ -105,7 +105,9 @@ module Jekyll
     #
     # Returns excerpt String
     def extract_excerpt(post_content)
-      separator     = site.config['excerpt_separator']
+      separator     = !post.excerpt_separator.empty? &&
+                       post.excerpt_separator ||
+                       site.config['excerpt_separator']
       head, _, tail = post_content.to_s.partition(separator)
 
       "" << head << "\n\n" << tail.scan(/^\[[^\]]+\]:.+$/).join("\n")

lib/jekyll/post.rb

@@ -23,6 +23,7 @@ module Jekyll
     ATTRIBUTES_FOR_LIQUID = EXCERPT_ATTRIBUTES_FOR_LIQUID + %w[
       content
       excerpt
+      excerpt_separator
     ]
 
     # Post name validator. Post filenames must be like:
@@ -118,6 +119,13 @@ module Jekyll
       data.fetch('title') { titleized_slug }
     end
 
+    # Public: the Post excerpt_separator, from the YAML Front-Matter or empty string
+    #
+    # Returns the post excerpt_separator
+    def excerpt_separator
+      data.fetch('excerpt_separator') { "" }
+    end
+
     # Turns the post slug into a suitable title
     def titleized_slug
       slug.split('-').select {|w| w.capitalize! || w }.join(' ')
@@ -307,7 +315,7 @@ module Jekyll
     end
 
     def generate_excerpt?
-      !(site.config['excerpt_separator'].to_s.empty?)
+      !site.config['excerpt_separator'].to_s.empty? || !excerpt_separator.empty?
     end
   end
 end

site/_docs/posts.md

@@ -171,6 +171,16 @@ your `excerpt_separator` to `""`.
 
 Also, as with any output generated by Liquid tags, you can pass the `| strip_html` flag to remove any html tags in the output. This is particularly helpful if you wish to output a post excerpt as a `meta="description"` tag within the post `head`, or anywhere else having html tags along with the content is not desirable.
 
+Additionally you are able to specify per-post `excerpt_separator` value if it is required just only the the selected post. Just specify the `excerpt_separator` with the same way as `excerpt` in the post's YAML head:
+
+    ---
+    excerpt_separator: <!--more-->
+    ---
+
+    Excerpt
+    <!--more-->
+    Out-of-excerpt
+
 ## Highlighting code snippets
 
 Jekyll also has built-in support for syntax highlighting of code snippets using

test/source/_posts/2015-01-08-post-excerpt-separator.markdown

@@ -0,0 +1,15 @@
+---
+layout: ~
+title:  Post Excerpt Separator
+excerpt_separator: "\n---\n"
+---
+
+First paragraph with [link ref][link].
+
+Second paragraph
+
+---
+
+Third paragraph
+
+[link]: http://www.jekyllrb.com/

test/test_generated_site.rb

@@ -14,7 +14,7 @@ class TestGeneratedSite < Test::Unit::TestCase
     end
 
     should "ensure post count is as expected" do
-      assert_equal 44, @site.posts.size
+      assert_equal 45, @site.posts.size
     end
 
     should "insert site.posts into the index" do

test/test_post.rb

@@ -397,6 +397,22 @@ class TestPost < Test::Unit::TestCase
           end
         end
 
+        context "with page's excerpt_separator setting" do
+          setup do
+            file = "2015-01-08-post-excerpt-separator.markdown"
+
+            @post.process(file)
+            @post.read_yaml(@source, file)
+            @post.transform
+          end
+
+          should "respect given separator" do
+            assert @post.excerpt.include?("First paragraph"), "contains first paragraph"
+            assert @post.excerpt.include?("Second paragraph"), "contains second paragraph"
+            assert !@post.excerpt.include?("Third paragraph"), "does not contains third paragraph"
+          end
+        end
+
         context "with custom excerpt" do
           setup do
             file = "2013-04-11-custom-excerpt.markdown"