Add ability to skip steps

[blink1073]

Add the ability to skip commands by name. Can be used to move pieces like check_links to another build to shorten release time.
Can be used to skip problematic sections (for example an npm-check in a Python package that would have caused problems).

[codecov-commenter]

Codecov Report

Merging #84 (1106f9e) into master (5b17742) will decrease coverage by 3.74%.
The diff coverage is 71.20%.

@@            Coverage Diff             @@
##           master      #84      +/-   ##
==========================================
- Coverage   89.03%   85.29%   -3.75%     
==========================================
  Files          15       16       +1     
  Lines        1697     1911     +214     
  Branches      198      240      +42     
==========================================
+ Hits         1511     1630     +119     
- Misses        118      190      +72     
- Partials       68       91      +23     

Impacted Files	Coverage Δ
jupyter_releaser/actions/draft_release.py	0.00% <0.00%> (ø)
jupyter_releaser/util.py	69.87% <51.42%> (-15.02%)	⬇️
jupyter_releaser/changelog.py	88.18% <62.50%> (-2.56%)	⬇️
jupyter_releaser/lib.py	74.28% <67.39%> (-3.46%)	⬇️
jupyter_releaser/npm.py	80.14% <70.37%> (-3.60%)	⬇️
jupyter_releaser/tee.py	82.05% <82.05%> (ø)
jupyter_releaser/tests/test_cli.py	96.54% <84.21%> (-1.30%)	⬇️
jupyter_releaser/__init__.py	100.00% <100.00%> (ø)
jupyter_releaser/cli.py	92.50% <100.00%> (+0.25%)	⬆️
jupyter_releaser/python.py	91.66% <100.00%> (+3.78%)	⬆️
... and 4 more

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