Add support for doctests (#1909)

Markdown files in the docs/ directory can now be annotated to turn
fenced C code blocks into unit tests. The recognized syntax is:

    [doctest]: # (name="test.c", test="run")

    ```c
    // unit test code
    ```

The commit also fixes the issues revealed during the initial
annotation.

JerryScript-DCO-1.0-Signed-off-by: Akos Kiss akiss@inf.u-szeged.hu

docs/10.EXT-REFERENCE-HANDLER.md

@@ -114,6 +114,8 @@ jerryx_handler_register_global (const jerry_char_t *name_p,
 
 **Example**
 
+[doctest]: # (test="compile")
+
 ```c
 #include "jerryscript.h"
 #include "jerryscript-ext/handler.h"
@@ -129,7 +131,8 @@ static const struct {
   { NULL, NULL }
 };
 
-static void register_common_functions ()
+static void
+register_common_functions (void)
 {
   jerry_value_t ret = jerry_create_undefined ();
 
@@ -139,7 +142,7 @@ static void register_common_functions ()
                                           common_functions[i].handler_p);
   }
 
-  return ret;
+  jerry_release_value (ret);
 }
 ```