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

docs/designers/language-modifiers/language-modifier-str-repeat.md

@@ -0,0 +1,14 @@
+# str_repeat
+
+Repeats the given value n times.
+
+## Basic usage
+```smarty
+{"hi"|str_repeat:2} # renders: hihi
+```
+
+## Parameters
+
+| Parameter | Type | Required | Description           |
+|-----------|------|----------|-----------------------|
+| 1         | int  | yes      | number of repetitions |