Add variable_name_expr to render's ParseTreeVisitor

Solves Shopify/theme-check#582

`icon` should be visited in the render tag for the following snippet:

```liquid
{% assign icon = 'warning' }
{% render 'icon' with icon %}
```

lib/liquid/tags/render.rb

@@ -31,7 +31,7 @@ module Liquid
 
     disable_tags "include"
 
-    attr_reader :template_name_expr, :attributes
+    attr_reader :template_name_expr, :variable_name_expr, :attributes
 
     def initialize(tag_name, markup, options)
       super
@@ -99,6 +99,7 @@ module Liquid
       def children
         [
           @node.template_name_expr,
+          @node.variable_name_expr,
         ] + @node.attributes.values
       end
     end