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
 |--------------------------------------------------------------------------
 |
diff --git a/system/core/Log.php b/system/core/Log.php
index 3b0a921..7572d2a 100644
--- a/system/core/Log.php
+++ b/system/core/Log.php
@@ -104,6 +104,8 @@
 
 		$this->_log_path = ($config['log_path'] !== '') ? $config['log_path'] : APPPATH.'logs/';
 
+		$this->_log_ext = ($config['log_file_extension'] !== '') ? $config['log_file_extension'] : $this->_log_ext;
+
 		file_exists($this->_log_path) OR mkdir($this->_log_path, DIR_WRITE_MODE, TRUE);
 
 		if ( ! is_dir($this->_log_path) OR ! is_really_writable($this->_log_path))
diff --git a/user_guide_src/source/changelog.rst b/user_guide_src/source/changelog.rst
index 893c3db..7f12ca8 100644
--- a/user_guide_src/source/changelog.rst
+++ b/user_guide_src/source/changelog.rst
@@ -55,6 +55,7 @@
    -  Updated *ip_address* database field lengths from 16 to 45 for supporting IPv6 address on :doc:`Trackback Library <libraries/trackback>` and :doc:`Captcha Helper <helpers/captcha_helper>`.
    -  Removed *cheatsheets* and *quick_reference* PDFs from the documentation.
    -  Added availability checks where usage of dangerous functions like ``eval()`` and ``exec()`` is required.
+   -  Added support for changing the file extension of CodeIgniter log files using $config['log_file_extension'].
 
 -  Helpers