Add support for CalVer

[mpanarin]

Closes #13

[codecov]

Codecov Report

Base: 80.43% // Head: 79.70% // Decreases project coverage by -0.73% ⚠️

Coverage data is based on head (8c6a8ba) compared to base (67d3d5f).
Patch coverage: 72.22% of modified lines in pull request are covered.

❗ Current head 8c6a8ba differs from pull request most recent head e2033a1. Consider uploading reports for the commit e2033a1 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #18      +/-   ##
==========================================
- Coverage   80.43%   79.70%   -0.74%     
==========================================
  Files          16       17       +1     
  Lines         184      202      +18     
==========================================
+ Hits          148      161      +13     
- Misses         36       41       +5     

Impacted Files	Coverage Δ
lib/Bumper/bumper.ex	86.95% <0.00%> (-10.61%)	⬇️
lib/Tasks/mix_bump.ex	100.00% <ø> (ø)
lib/Bumper/calver_formatting.ex	100.00% <100.00%> (ø)

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