introduce an exclude config option

This is a YAML array for files in the root directory,
which should be excluded to be processed into '_site'.
This can be useful for README, Rakefiles etc.

lib/jekyll/site.rb

@@ -1,7 +1,7 @@
 module Jekyll
 
   class Site
-    attr_accessor :config, :layouts, :posts, :categories
+    attr_accessor :config, :layouts, :posts, :categories, :exclude
     attr_accessor :source, :dest, :lsi, :pygments, :permalink_style
 
     # Initialize the site
@@ -17,6 +17,8 @@ module Jekyll
       self.pygments        = config['pygments']
       self.permalink_style = config['permalink'].to_sym
 
+      self.exclude         = config['exclude']
+
       self.reset
       self.setup
     end
@@ -230,7 +232,7 @@ module Jekyll
       entries = entries.reject do |e|
         unless ['_posts', '.htaccess'].include?(e)
           # Reject backup/hidden
-          ['.', '_', '#'].include?(e[0..0]) or e[-1..-1] == '~'
+          ['.', '_', '#'].include?(e[0..0]) or e[-1..-1] == '~' or self.exclude.include?(e)
         end
       end
     end