Deprecate CI_Input::is_cli_request() and add common function is_cli() to replace it

Calls to this function are often needed before the Input library is available
diff --git a/system/core/Common.php b/system/core/Common.php
index 56008ef..c008bd5 100644
--- a/system/core/Common.php
+++ b/system/core/Common.php
@@ -355,6 +355,24 @@
 
 // ------------------------------------------------------------------------
 
+if ( ! function_exists('is_cli'))
+{
+
+	/**
+	 * Is CLI?
+	 *
+	 * Test to see if a request was made from the command line.
+	 *
+	 * @return 	bool
+	 */
+	function is_cli()
+	{
+		return (PHP_SAPI === 'cli' OR defined('STDIN'));
+	}
+}
+
+// ------------------------------------------------------------------------
+
 if ( ! function_exists('show_error'))
 {
 	/**