Add subclass hook for additional data transformation to Presto cursor class.

[jezdez]

This is a follow-up of #82 which proposed a backward-incompatible change
for the strucutre of data returned by the Presto cursor and simplifies
doing additional data transformation without having to override the majority
of the response processing method.

The purpose of this is being able to extend Redash's (https://redash.io)
Presto query runner to show more human-readable query results.

I've signed the Dropbox CLA.

[codecov-io]

Codecov Report

Merging #222 into master will decrease coverage by 0.06%.
The diff coverage is 88.88%.

@@            Coverage Diff             @@
##           master     #222      +/-   ##
==========================================
- Coverage   93.94%   93.88%   -0.07%     
==========================================
  Files          14       14              
  Lines        1487     1504      +17     
  Branches      159      159              
==========================================
+ Hits         1397     1412      +15     
- Misses         64       66       +2     
  Partials       26       26

Impacted Files	Coverage Δ
pyhive/presto.py	93.7% <100%> (+0.08%)	⬆️
pyhive/tests/test_presto.py	98.4% <86.66%> (-1.6%)	⬇️

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[jezdez]

@gsilk Hey there, I was wondering if there is anything else needed for this to be reviewed?

[jezdez]

Is anyone else interested in reviewing this at Dropbox?

[CLAassistant]

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