Implement package versions based on Git tags

[fghaas]

• Use setuptools-scm so that setup.py can discern version numbers from Git tags, as recommended by option 7 in: https://packaging.python.org/guides/single-sourcing-package-version/
• Define a __version__ property as recommended by (deferred) PEP 396: https://www.python.org/dev/peps/pep-0396/
• Add a -V/--version option to the CLI

[codecov-io]

Codecov Report

Merging #30 into master will increase coverage by 0.01%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #30      +/-   ##
==========================================
+ Coverage   98.33%   98.35%   +0.01%     
==========================================
  Files           8        9       +1     
  Lines         481      485       +4     
==========================================
+ Hits          473      477       +4     
  Misses          8        8

Impacted Files	Coverage Δ
olxutils/__init__.py	100% <100%> (ø)
olxutils/cli.py	95.23% <100%> (+0.11%)	:arrow_up:

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