Add namespace support in options

[HyukjinKwon]

This PR adds the support for options to be set like attributes.

>>> import databricks.koalas as ks
>>> ks.options.display.max_rows
1000
>>> ks.options.display.max_rows = 10
>>> ks.options.display.max_rows
10

[codecov-io]

Codecov Report

Merging #785 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #785      +/-   ##
==========================================
+ Coverage   94.28%   94.32%   +0.03%     
==========================================
  Files          32       32              
  Lines        5776     5812      +36     
==========================================
+ Hits         5446     5482      +36     
  Misses        330      330

Impacted Files	Coverage Δ
databricks/koalas/__init__.py	85% <100%> (ø)	⬆️
databricks/koalas/config.py	87.35% <100%> (+8.92%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 216e524...d740c34. Read the comment docs.

[ueshin]

This is great!

[HyukjinKwon]

Thanks for detailed review @ueshin always!

[softagram-bot]

Softagram Impact Report for pull/785 (head commit: d740c34)

⭐ Change Overview

(Open in Softagram Desktop for full details)

📄 Full report

• Permalink: Full report for pull/785

Impact Report explained. Give feedback on this report to [email protected]

[HyukjinKwon]

Today is the release date. I am merging it in