Implementing sass_free_folder_context() function.

Although sass_interface.h header contains the prototype of
void sass_free_folder_context(struct sass_folder_context *)
function, sass_interface.cpp hadn't implemented it.
This had caused link errors.

Author: dahlia

sass_interface.cpp

@@ -37,6 +37,14 @@ extern "C" {
   sass_folder_context* sass_new_folder_context()
     { return (sass_folder_context*) calloc(1, sizeof(sass_folder_context)); }
 
+  void sass_free_folder_context(sass_folder_context* ctx)
+  {
+    if (ctx->output_path) free(ctx->output_path);
+    if (ctx->error_message) free(ctx->error_message);
+
+    free(ctx);
+  }
+
   static char* process_document(Sass::Document& doc, int style)
   {
     using namespace Sass;