Implement DatetimeIndex.month_name() and DatetimeIndex.day_name()

[xinrong-meng]

>>> idx = ks.date_range(start='2018-01', freq='M', periods=3)
>>> idx.month_name()
Index(['January', 'February', 'March'], dtype='object')

>>> idx = ks.date_range(start='2018-01-01', freq='D', periods=3)
>>> idx.day_name()
Index(['Monday', 'Tuesday', 'Wednesday'], dtype='object')

[codecov-io]

Codecov Report

Merging #2086 (042725a) into master (b32203b) will decrease coverage by 3.78%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2086      +/-   ##
==========================================
- Coverage   95.28%   91.49%   -3.79%     
==========================================
  Files          57       57              
  Lines       13227    13119     -108     
==========================================
- Hits        12603    12003     -600     
- Misses        624     1116     +492     

Impacted Files	Coverage Δ
databricks/koalas/missing/indexes.py	100.00% <ø> (ø)
databricks/koalas/indexes/datetimes.py	96.00% <100.00%> (-2.35%)	⬇️
databricks/koalas/tests/indexes/test_datetime.py	97.22% <100.00%> (-2.78%)	⬇️
databricks/koalas/plot/plotly.py	15.78% <0.00%> (-81.06%)	⬇️
...bricks/koalas/tests/plot/test_frame_plot_plotly.py	23.33% <0.00%> (-76.67%)	⬇️
...ricks/koalas/tests/plot/test_series_plot_plotly.py	26.92% <0.00%> (-71.26%)	⬇️
databricks/koalas/__init__.py	74.66% <0.00%> (-17.34%)	⬇️
databricks/koalas/missing/__init__.py	84.61% <0.00%> (-15.39%)	⬇️
databricks/koalas/typedef/typehints.py	79.88% <0.00%> (-14.53%)	⬇️
databricks/conftest.py	92.72% <0.00%> (-7.28%)	⬇️
... and 28 more

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[ueshin]

LGTM.

[itholic]

Otherwise, LGTM!

[itholic]

@xinrong-databricks , Let me just merge this to include this change to 1.7.0 release.

Will fix #2086 (comment).