Provide --update option as comma separated list

[sbidoul]

This will better match the natural expectation for extras.

[codecov]

Codecov Report

Merging #33 into master will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #33      +/-   ##
==========================================
+ Coverage   94.05%   94.11%   +0.06%     
==========================================
  Files          14       14              
  Lines         740      748       +8     
  Branches      134      138       +4     
==========================================
+ Hits          696      704       +8     
  Misses         28       28              
  Partials       16       16              

Impacted Files	Coverage Δ
src/pip_deepfreeze/__main__.py	94.87% <100.00%> (ø)
src/pip_deepfreeze/utils.py	96.72% <100.00%> (+0.49%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 483a575...56896f5. Read the comment docs.