Fix handling of default version bump

[blink1073]

Provide reasonable defaults for version bump but allow it to be overridden.
Needed for jupyterlab/jupyterlab#10340 (jupyterlab/jupyterlab#10340)

[codecov-commenter]

Codecov Report

Merging #57 (0678061) into master (9129099) will decrease coverage by 0.64%.
The diff coverage is 72.72%.

@@            Coverage Diff             @@
##           master      #57      +/-   ##
==========================================
- Coverage   88.51%   87.86%   -0.65%     
==========================================
  Files          15       15              
  Lines        1715     1755      +40     
  Branches      205      215      +10     
==========================================
+ Hits         1518     1542      +24     
- Misses        122      132      +10     
- Partials       75       81       +6     

Impacted Files	Coverage Δ
jupyter_releaser/util.py	82.19% <37.50%> (-2.71%)	⬇️
jupyter_releaser/lib.py	75.29% <66.66%> (-1.07%)	⬇️
jupyter_releaser/npm.py	78.83% <70.00%> (-1.79%)	⬇️
jupyter_releaser/tests/test_cli.py	97.64% <91.66%> (-0.20%)	⬇️
jupyter_releaser/__init__.py	100.00% <100.00%> (ø)
jupyter_releaser/cli.py	92.37% <100.00%> (+0.09%)	⬆️

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