Added support for changing the default log file extension from .php to whatever is preferred. example (.log)

This is a follow up to this pull request.

https://github.com/EllisLab/CodeIgniter/pull/2243
diff --git a/application/config/config.php b/application/config/config.php
index 415474e..6f597b1 100644
--- a/application/config/config.php
+++ b/application/config/config.php
@@ -226,6 +226,17 @@
 
 /*
 |--------------------------------------------------------------------------
+| Log File Extension
+|--------------------------------------------------------------------------
+|
+| Leave this BLANK unless you would like to set something other than the default
+| 'php'. For example you could change it to 'log'.
+|
+*/
+$config['log_file_extension'] = '';
+
+/*
+|--------------------------------------------------------------------------
 | Date Format for Logs
 |--------------------------------------------------------------------------
 |