add flatten filter

lib/liquid/standardfilters.rb

@@ -101,7 +101,7 @@ module Liquid
       ary = InputIterator.new(input)
       if property.nil?
         ary.sort
-      elsif ary.first.respond_to?('[]'.freeze) and !ary.first[property].nil?
+      elsif ary.first.respond_to?('[]'.freeze) && !ary.first[property].nil?
         ary.sort {|a,b| a[property] <=> b[property] }
       elsif ary.first.respond_to?(property)
         ary.sort {|a,b| a.send(property) <=> b.send(property) }
@@ -127,6 +127,11 @@ module Liquid
       end
     end
 
+    # flatten the given input
+    def flatten(input)
+      Array(input).flatten
+    end
+
     # Replace occurrences of a string with another
     def replace(input, string, replacement = ''.freeze)
       input.to_s.gsub(string, replacement.to_s)

test/integration/standard_filter_test.rb

@@ -149,6 +149,16 @@ class StandardFiltersTest < Test::Unit::TestCase
       "thing" => { "foo" => [ { "bar" => 42 }, { "bar" => 17 } ] }
   end
 
+  def test_flatten
+    assert_equal [1,2,3,4], @filters.flatten([1,2,3,4])
+    assert_equal [1,2,3,4], @filters.flatten([[1,2,3,4]])
+    assert_equal [1,2,3,4], @filters.flatten([[1],[2],[3],[4]])
+    assert_equal [1], @filters.flatten(1)
+
+    assert_template_result '1234', "{{ ary | flatten | flatten }}",
+      'ary' => [[1],2,[3],4]
+  end
+
   def test_sort_calls_to_liquid
     t = TestThing.new
     Liquid::Template.parse('{{ foo | sort: "whatever" }}').render("foo" => [t])