Make branch and version spec optional

[blink1073]

Fixes #222

[codecov-commenter]

Codecov Report

Merging #223 (fa8749a) into master (18d0a59) will increase coverage by 0.04%.
The diff coverage is 44.44%.

@@            Coverage Diff             @@
##           master     #223      +/-   ##
==========================================
+ Coverage   85.06%   85.11%   +0.04%     
==========================================
  Files          19       19              
  Lines        2303     2304       +1     
  Branches      292      290       -2     
==========================================
+ Hits         1959     1961       +2     
- Misses        246      247       +1     
+ Partials       98       96       -2     

Impacted Files	Coverage Δ
jupyter_releaser/actions/common.py	0.00% <0.00%> (ø)
jupyter_releaser/actions/generate-changelog.py	0.00% <0.00%> (ø)
jupyter_releaser/changelog.py	87.27% <100.00%> (+0.11%)	⬆️
jupyter_releaser/lib.py	83.94% <100.00%> (+0.04%)	⬆️
jupyter_releaser/util.py	73.30% <100.00%> (+1.39%)	⬆️

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[jtpio]

Thanks!

[jtpio]

Maybe we can also update the docs to mention the default behavior as a follow-up.