Implements truncate for DataFrame

[itholic]

This PR proposes the DataFrame.truncate().

>>> df = ks.DataFrame({'A': ['a', 'b', 'c', 'd', 'e'],
...                    'B': ['f', 'g', 'h', 'i', 'j'],
...                    'C': ['k', 'l', 'm', 'n', 'o']},
...                   index=[1, 2, 3, 4, 5])
>>> df
   A  B  C
1  a  f  k
2  b  g  l
3  c  h  m
4  d  i  n
5  e  j  o

>>> df.truncate(before=2, after=4)
   A  B  C
2  b  g  l
3  c  h  m
4  d  i  n

The columns of a DataFrame can be truncated.

>>> df.truncate(before="A", after="B", axis="columns")
   A  B
1  a  f
2  b  g
3  c  h
4  d  i
5  e  j

For Series, only rows can be truncated.

>>> df['A'].truncate(before=2, after=4)
2    b
3    c
4    d
Name: A, dtype: object

[codecov-io]

Codecov Report

Merging #1408 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1408   +/-   ##
=======================================
  Coverage   95.01%   95.01%           
=======================================
  Files          34       34           
  Lines        7944     7949    +5     
=======================================
+ Hits         7548     7553    +5     
  Misses        396      396           

Impacted Files	Coverage Δ
databricks/koalas/missing/frame.py	100.00% <ø> (ø)
databricks/koalas/series.py	96.92% <ø> (-0.05%)	⬇️
databricks/koalas/frame.py	96.61% <100.00%> (-0.01%)	⬇️
databricks/koalas/generic.py	96.96% <100.00%> (+0.21%)	⬆️

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[HyukjinKwon]

Looks good otherwise.