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/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))