Allow a custom 'layouts' directory

* Add 'layouts' option to change the dir from '_layouts' to anything relative
  to the source directory
* Add cucumber scenario for testing an alternative directory '_theme'
* Closes jekyll#563

features/site_configuration.feature

@@ -143,3 +143,24 @@ Feature: Site configuration
     Then the _site directory should exist
     And I should see ".DS_Store" in "_site/.gitignore"
     And the "_site/.htaccess" file should not exist
+
+  Scenario: Using a different layouts directory
+    Given I have a _theme directory
+    And I have a page theme that contains "Page Layout: {{ site.posts.size }} on {{ site.time | date: "%Y-%m-%d" }}"
+    And I have a post theme that contains "Post Layout: {{ content }}"
+    And I have an "index.html" page with layout "page" that contains "site index page"
+    And I have a configuration file with:
+      | key         | value        |
+      | time        | 2010-01-01   |
+      | future      | true         |
+      | layouts     | _theme       |
+    And I have a _posts directory
+    And I have the following posts:
+      | title     | date       | layout  | content                                |
+      | entry1    | 12/31/2007 | post    | content for entry1.                    |
+      | entry2    | 01/31/2020 | post    | content for entry2.                    |
+    When I run jekyll
+    Then the _site directory should exist
+    And I should see "Page Layout: 2 on 2010-01-01" in "_site/index.html"
+    And I should see "Post Layout: <p>content for entry1.</p>" in "_site/2007/12/31/entry1.html"
+    And I should see "Post Layout: <p>content for entry2.</p>" in "_site/2020/01/31/entry2.html"

features/step_definitions/jekyll_steps.rb

@@ -39,6 +39,13 @@
   end
 end
 
+Given /^I have a (.*) theme that contains "(.*)"$/ do |layout, text|
+  File.open(File.join('_theme', layout + '.html'), 'w') do |f|
+    f.write(text)
+    f.close
+  end
+end
+
 Given /^I have an? (.*) directory$/ do |dir|
   FileUtils.mkdir_p(dir)
 end

lib/jekyll.rb

@@ -59,6 +59,7 @@ module Jekyll
     'source'       => Dir.pwd,
     'destination'  => File.join(Dir.pwd, '_site'),
     'plugins'      => File.join(Dir.pwd, '_plugins'),
+    'layouts'      => '_layouts',
 
     'future'       => true,
     'lsi'          => false,

lib/jekyll/site.rb

@@ -106,7 +106,7 @@ def read
     #
     # Returns nothing.
     def read_layouts(dir = '')
-      base = File.join(self.source, dir, "_layouts")
+      base = File.join(self.source, dir, self.config['layouts'])
       return unless File.exists?(base)
       entries = []
       Dir.chdir(base) { entries = filter_entries(Dir['*.*']) }