Add ParseTreeVisitor to Echo tag

This fixes theme-check#218, wherein variables used in echo tags are not
considered used by the linter. It is because our visitor doesn't see the
:variable_lookup's in the echo tag since the children array is empty.
But this array is empty because it is swallowed by the @variable.

lib/liquid/tags/echo.rb

@@ -12,6 +12,8 @@ module Liquid
   #   {% echo user | link %}
   #
   class Echo < Tag
+    attr_reader :variable
+
     def initialize(tag_name, markup, parse_context)
       super
       @variable = Variable.new(markup, parse_context)
@@ -20,6 +22,12 @@ module Liquid
     def render(context)
       @variable.render_to_output_buffer(context, +'')
     end
+
+    class ParseTreeVisitor < Liquid::ParseTreeVisitor
+      def children
+        [@node.variable]
+      end
+    end
   end
 
   Template.register_tag('echo', Echo)