add weight in tree model output

[guolinke]

fix #2037

[StrikerRUS]

@guolinke I can update plotting part if you don't mind.

LightGBM/python-package/lightgbm/plotting.py

Lines 392 to 396 in 611cf5d

	for info in show_info:
	if info in {'split_gain', 'internal_value'}:
	label += r'\n{0}: {1}'.format(info, _float2str(root[info], precision))
	elif info == 'internal_count':
	label += r'\n{0}: {1}'.format(info, root[info])

[guolinke]

@StrikerRUS sure, thanks very much!

[StrikerRUS]

@guolinke Just updated plotting module in Python in the latest commit.

[StrikerRUS]

ping @jameslamb and @Laurae2 for R-package updates

https://github.com/microsoft/LightGBM/blob/master/R-package/R/lgb.model.dt.tree.R
https://github.com/microsoft/LightGBM/blob/master/R-package/R/lgb.interprete.R

[guolinke]

@StrikerRUS could this PR merge first? then a new PR for R package could be proposed.

[StrikerRUS]

@guolinke Yeah, sure!

[jameslamb]

Created #2281 to track the R work, agree with the decision that was made to merge this and come back to R.