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)