Add Since Last Stable Option for Changelog

[jtpio]

Add an option to use PRs with activity since the last stable git tag. This is useful for creating the first alpha release or first final release in a release cycle.

[codecov-commenter]

Codecov Report

Merging #147 (90977b9) into master (aefad8b) will increase coverage by 0.12%.
The diff coverage is 91.66%.

@@            Coverage Diff             @@
##           master     #147      +/-   ##
==========================================
+ Coverage   85.11%   85.24%   +0.12%     
==========================================
  Files          17       17              
  Lines        2116     2141      +25     
  Branches      257      260       +3     
==========================================
+ Hits         1801     1825      +24     
- Misses        223      224       +1     
  Partials       92       92              

Impacted Files	Coverage Δ
jupyter_releaser/lib.py	83.88% <75.00%> (+0.09%)	⬆️
jupyter_releaser/changelog.py	88.23% <85.71%> (-0.06%)	⬇️
jupyter_releaser/cli.py	90.69% <100.00%> (ø)
jupyter_releaser/tests/test_cli.py	99.17% <100.00%> (+<0.01%)	⬆️
jupyter_releaser/tests/test_functions.py	98.30% <100.00%> (+0.20%)	⬆️

---

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 aefad8b...90977b9. Read the comment docs.

[blink1073]

Thank you!