[R-package] add flag of displaying train loss for lgb.cv()

[mayer79]

In #1105, it was suggested to add the option eval_train_metric to lgb.cv() in order to see not only CV scores but also training scores. This is very useful to monitor overfitting.

In Python, it was implemented in #2089.

I would love to see something similar for R.

[jameslamb]

Thanks very much for the suggestion @mayer79 ! I support it.

Are you interested in contributing this? I'd be happy to answer any questions you have about where to make changes.