Allow filters in assign.

A simple attempt, but basically it shifts the parsing of the right hand side of the assign from a simple context lookup to using the Variable parser (that includes filters). Fixes #79.

test/liquid/assign_test.rb

@@ -12,4 +12,10 @@ class AssignTest < Test::Unit::TestCase
                            '{% assign foo = values %}.{{ foo[1] }}.',
                            'values' => %w{foo bar baz})
   end
+  
+  def test_assign_with_filter
+    assert_template_result('.bar.',
+                           '{% assign foo = values | split: "," %}.{{ foo[1] }}.',
+                           'values' => "foo,bar,baz")
+  end
 end # AssignTest