Add mean value for reduce option
Add mean value for reduce option

Add feature based on #5939 .
keisuke-umezawa/chainer,keisuke-umezawa/chainer,niboshi/chainer,hvy/chainer,hvy/chainer,okuta/chainer,chainer/chainer,okuta/chainer,hvy/chainer,tkerola/chainer,okuta/chainer,niboshi/chainer,wkentaro/chainer,okuta/chainer,chainer/chainer,keisuke-umezawa/chainer,wkentaro/chainer,keisuke-umezawa/chainer,hvy/chainer,wkentaro/chainer,chainer/chainer,chainer/chainer,pfnet/chainer,niboshi/chainer,niboshi/chainer,wkentaro/chainer