Look for plugins under the source directory

When generating the site, Jekyll will now look for plugins under the
source directory by default. The plugin location can still be changed in
_config.yml

lib/jekyll.rb

@@ -58,7 +58,7 @@ module Jekyll
 
     'source'       => Dir.pwd,
     'destination'  => File.join(Dir.pwd, '_site'),
-    'plugins'      => File.join(Dir.pwd, '_plugins'),
+    'plugins'      => '_plugins',
     'layouts'      => '_layouts',
 
     'future'       => true,