Add Liquid::ParseContext#parse_expression for liquid-c node disabling (#1333)

We would like to be able to disable liquid-c VM rendering at runtime,
but right now expression parsing is done using Expression.parse, which
isn't aware of the parse context.  That prevents us from conditionally
compiling to VM code based on a parse option.

lib/liquid/parse_context.rb

@@ -23,6 +23,10 @@ module Liquid
       Liquid::BlockBody.new
     end
 
+    def parse_expression(markup)
+      Expression.parse(markup)
+    end
+
     def partial=(value)
       @partial = value
       @options = value ? partial_options : @template_options