Add NamedAgg

[charlesdong1991]

This is equivalent to nested renaming, so like pandas, we have koalas.NamedAgg now for this:

        >>> aggregated = df.groupby('A').agg(b_max=ks.NamedAgg(column='B', aggfunc='max'))
        >>> aggregated
             b_max
        A
        1        2
        2        4

Resolves #823

[codecov-io]

Codecov Report

Merging #911 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #911      +/-   ##
=========================================
+ Coverage   94.28%   94.3%   +0.01%     
=========================================
  Files          34      34              
  Lines        6265    6355      +90     
=========================================
+ Hits         5907    5993      +86     
- Misses        358     362       +4

Impacted Files	Coverage Δ
databricks/koalas/__init__.py	82.92% <100%> (-2.08%)	⬇️
databricks/koalas/groupby.py	91.18% <100%> (+0.04%)	⬆️
databricks/conftest.py	95.74% <0%> (-2.13%)	⬇️
databricks/koalas/generic.py	95.07% <0%> (-0.5%)	⬇️
databricks/koalas/missing/groupby.py	100% <0%> (ø)	⬆️
databricks/koalas/missing/frame.py	100% <0%> (ø)	⬆️
databricks/koalas/missing/series.py	100% <0%> (ø)	⬆️
databricks/koalas/frame.py	95.98% <0%> (+0.04%)	⬆️
... and 4 more

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[HyukjinKwon]

Can we add it to __all__?

[charlesdong1991]

thanks! added!

[softagram-bot]

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[HyukjinKwon]

Thanks @charlesdong1991!