Implement expanding().count() in Series and Frame

[HyukjinKwon]

This PR implements expanding().count():

>>> import databricks.koalas as ks
>>> ks.Series([2, 3, float("nan"), 10]).expanding().count()
0    1.0
1    2.0
2    2.0
3    3.0
Name: 0, dtype: float64
>>> ks.DataFrame({'a': [1, float('nan'), 3], 'b': [1.0, 2.0, 3.0]}).expanding().count()
     a    b
0  1.0  1.0
1  1.0  2.0
2  2.0  3.0

Relates to #977

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[HyukjinKwon]

I'll fix this one nicer when I add count into DataFrameGroupBy and SeriesGroupBy in a separate PR.

[ueshin]

LGTM, pending tests.

[codecov-io]

Codecov Report

Merging #978 into master will increase coverage by 0.02%.
The diff coverage is 98.11%.

@@            Coverage Diff             @@
##           master     #978      +/-   ##
==========================================
+ Coverage   94.82%   94.84%   +0.02%     
==========================================
  Files          34       34              
  Lines        6489     6519      +30     
==========================================
+ Hits         6153     6183      +30     
  Misses        336      336

Impacted Files	Coverage Δ
databricks/koalas/missing/window.py	100% <100%> (ø)	⬆️
databricks/koalas/generic.py	96.17% <100%> (ø)	⬆️
databricks/koalas/window.py	92.85% <97.05%> (+6.01%)	⬆️

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[HyukjinKwon]

Thanks @ueshin !