[python] add flag of displaying train loss for lgb.cv()

[matsuken92]

Adding flag on lgb.cv() for displaying train loss during training iteration.

[StrikerRUS]

Closed #1105.
Glad to see how very old issues are closing! 😃

[StrikerRUS]

I began to doubt in new parameter name. Seems that I was wrong when suggested to rename disp_* -> show_*. Both these words describe some appearance. There are even the following remark in show_stdv description:

Results are not affected by this parameter, and always contain std.

In contrast, new parameter affects performance because requires additional metric computation for train dataset.
I think we need a better word. What about calc(ulate), comp(ute), eval(uate)? I personally prefer eval.

BTW, loss should be replaced with metric for the clarity too.

[StrikerRUS]

@matsuken92 Thanks a lot for this nice feature! I'm OK to merge after fixing two very minor nitpicks below.

[henry0312]

I'm sorry for rate response but I didn't have much time because of moving.
I'll check today.