[WIP] Add versioneer and PyPI deployment

[betatim]

We will get version tags based on git commits or git tags.

[codecov-io]

Codecov Report

Merging #24 into master will decrease coverage by 30.1%.
The diff coverage is 41.78%.

@@             Coverage Diff             @@
##           master      #24       +/-   ##
===========================================
- Coverage   91.44%   61.33%   -30.11%     
===========================================
  Files           4        4               
  Lines         187      463      +276     
===========================================
+ Hits          171      284      +113     
- Misses         16      179      +163

Impacted Files	Coverage Δ
cornerstone_widget/__init__.py	85.71% <100%> (+5.71%)	⬆️
cornerstone_widget/_version.py	41.15% <41.15%> (-58.85%)	⬇️

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