Add Configuration.from & use in Jekyll.configuration.

This process streamlines the creation of new configurations. Creating a new
site will choke if not all the correct options are given.
Configuration.from will ensure the overrides have all string keys and
ensures all the common issues & defaults are in place so a Site can be
created.

A common use:

    config = Configuration.from({ 'permalink' => '/:title/' }) # etc
    site = Jekyll::Site.new(config)

lib/jekyll.rb

@@ -98,15 +98,14 @@ module Jekyll
     #            list of option names and their defaults.
     #
     # Returns the final configuration Hash.
-    def configuration(override = {})
-      config = Configuration[Configuration::DEFAULTS]
-      override = Configuration[override].stringify_keys
+    def configuration(override = Hash.new)
+      config = Configuration.new
       unless override.delete('skip_config_files')
         config = config.read_config_files(config.config_files(override))
       end
 
       # Merge DEFAULTS < _config.yml < override
-      config = Utils.deep_merge_hashes(config, override).stringify_keys
+      config = Configuration.from Utils.deep_merge_hashes(config, override).stringify_keys
       set_timezone(config['timezone']) if config['timezone']
 
       config

lib/jekyll/configuration.rb

@@ -74,6 +74,23 @@ module Jekyll
       }
     }].freeze
 
+    class << self
+      # Static: Produce a Configuration ready for use in a Site.
+      # It takes the input, fills in the defaults where values do not
+      # exist, and patches common issues including migrating options for
+      # backwards compatiblity. Except where a key or value is being fixed,
+      # the user configuration will override the defaults.
+      #
+      # user_config - a Hash or Configuration of overrides.
+      #
+      # Returns a Configuration filled with defaults and fixed for common
+      # problems and backwards-compatibility.
+      def from(user_config)
+        Utils.deep_merge_hashes(DEFAULTS, Configuration[user_config].stringify_keys).
+          fix_common_issues.backwards_compatibilize.add_default_collections
+      end
+    end
+
     # Public: Turn all keys into string
     #
     # Return a copy of the hash where all its keys are strings
@@ -237,7 +254,7 @@ module Jekyll
             " as a list of comma-separated values."
           config[option] = csv_to_array(config[option])
         end
-        config[option].map!(&:to_s)
+        config[option].map!(&:to_s) if config[option]
       end
 
       if (config['kramdown'] || {}).key?('use_coderay')