Set _evals_result so that lgb.plot_metric can be used

[Y-oHr-N]

This PR closes #75.

[codecov-io]

Codecov Report

Merging #77 into master will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #77      +/-   ##
==========================================
+ Coverage   96.27%   96.33%   +0.05%     
==========================================
  Files           8        8              
  Lines         564      573       +9     
==========================================
+ Hits          543      552       +9     
  Misses         21       21              

Impacted Files	Coverage Δ
optgbm/sklearn.py	96.44% <100.00%> (+0.05%)	⬆️
tests/test_sklearn.py	99.44% <100.00%> (+0.01%)	⬆️

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[Y-oHr-N]

import optgbm as lgb
from matplotlib import pyplot as plt
from sklearn.datasets import load_iris

X, y = load_iris(return_X_y=True)
clf = lgb.LGBMClassifier(random_state=0)

clf.fit(X, y)

lgb.plot_metric(clf)

plt.show()