Change config key functions_asis to function and add documentation

Author: kenjis

README.md

@@ -71,7 +71,7 @@ Remove comment marks below and fix the path for `Autoloader.php`:
 $this->load->library('twig');
 ~~~
 
-You can override the default configration:
+You can override the default configuration:
 
 ~~~php
 $config = [
@@ -126,6 +126,20 @@ $twig = $this->twig->getTwig();
 
 Some helpers are added the functionality of auto-escaping for security.
 
+### Adding Your Functions
+
+You can add your functions with configuration:
+
+~~~php
+$config = [
+	'functions' => ['my_helper'],
+	'functions_safe' => ['my_safe_helper'],
+];
+$this->load->library('twig', $config);
+~~~
+
+If your filter explicitly outputs HTML code, you will want the raw output to be printed. In such a case, use `functions_safe`, and you have to make sure the output of the function is XSS free.
+
 ### Reference
 
 #### Documentation

libraries/Twig.php

@@ -50,11 +50,11 @@ public function __construct($params = [])
 
 		$this->config = array_merge($this->config, $params);
 
-		if (isset($params['functions_asis']))
+		if (isset($params['functions']))
 		{
 			$this->functions_asis = 
 				array_unique(
-					array_merge($this->functions_asis, $params['functions_asis'])
+					array_merge($this->functions_asis, $params['functions'])
 				);
 		}
 		if (isset($params['functions_safe']))

tests/libraries/TwigTest.php

@@ -80,7 +80,7 @@ public function testFunctionAsIs()
     {
         $obj = new Twig([
             'paths' => __DIR__ . '/../templates/',
-            'functions_asis' => ['md5'],
+            'functions' => ['md5'],
             'cache' => false,
         ]);