Add Utils.has_yaml_header? so we can check the front matter existence in one place

lib/jekyll/utils.rb

@@ -1,6 +1,6 @@
 module Jekyll
   module Utils
-    class << self
+    extend self
 
     # Merges a master hash with another hash, recursively.
     #
@@ -95,6 +95,12 @@ module Jekyll
       raise Errors::FatalException.new("Invalid date '#{input}': " + msg)
     end
 
+    # Determines whether a given file has
+    #
+    # Returns true if the YAML front matter is present.
+    def has_yaml_header?(file)
+      !!(File.open(file, 'rb') { |f| f.read(5) } =~ /\A---\r?\n/)
     end
+
   end
 end