adds sliding_window iter postfix ++
adds sliding_window iter postfix ++

jkhoogland/cppitertools,jkhoogland/cppitertools,jkhoogland/cppitertools,ryanhaining/cppitertools,tempbottle/cppitertools,ryanhaining/cppitertools,tempbottle/cppitertools,tempbottle/cppitertools,ryanhaining/cppitertools