Implemented support for substr, implode and json_encode as modifiers. (#940)

* Implemented support for substr, implode and json_encode as modifiers. Fixes #939
* Added split and join in favor of explode and implode modifiers.
* Documented all available modifiers

src/Compile/Modifier/JsonEncodeModifierCompiler.php

@@ -0,0 +1,14 @@
+<?php
+
+namespace Smarty\Compile\Modifier;
+
+/**
+ * Smarty json_encode modifier plugin
+ */
+class JsonEncodeModifierCompiler extends Base {
+
+	public function compile($params, \Smarty\Compiler\Template $compiler) {
+		return 'json_encode(' . $params[0] . (isset($params[1]) ? ', (int) ' . $params[1] : '') . ')';
+	}
+
+}