Add support for indented link references on excerpt

Excerpt link reference extraction is missing all the indented references
at the bottom of the page. Markdown specify that those can be indented up
to three spaces.

lib/jekyll/excerpt.rb

@@ -118,7 +118,7 @@ module Jekyll
       if tail.empty?
         head
       else
-        "" << head << "\n\n" << tail.scan(%r!^\[[^\]]+\]:.+$!).join("\n")
+        "" << head << "\n\n" << tail.scan(%r!^ {0,3}\[[^\]]+\]:.+$!).join("\n")
       end
     end
   end