feat: add signoff parameter to commit command

[manang]

command to sign off the commit, equivalent to git commit -s

This feature permits to execute "git commit -s" command

Description

when cz commit --signoff is launched, the command git commit -s is executed

Checklist

• Add test cases to all the changes you introduce
• Run ./script/format and ./script/test locally to ensure this change passes linter check and test
• Test the changes on the local machine manually
• Update the documentation for the changes

Expected behavior

Signed-off-by message added to the footer of the commit

[codecov]

Codecov Report

Merging #439 (7b7b0c9) into master (ca0e4bc) will increase coverage by 0.01%.
The diff coverage is 100.00%.

❗ Current head 7b7b0c9 differs from pull request most recent head 5e22040. Consider uploading reports for the commit 5e22040 to get more accurate results

@@            Coverage Diff             @@
##           master     #439      +/-   ##
==========================================
+ Coverage   97.90%   97.92%   +0.01%     
==========================================
  Files          39       39              
  Lines        1384     1395      +11     
==========================================
+ Hits         1355     1366      +11     
  Misses         29       29              

Flag	Coverage Δ
unittests	97.92% <100.00%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
commitizen/cli.py	97.61% <ø> (ø)
commitizen/__version__.py	100.00% <100.00%> (ø)
commitizen/commands/commit.py	98.41% <100.00%> (+0.07%)	⬆️
commitizen/commands/version.py	100.00% <100.00%> (ø)

---

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 976d5d2...5e22040. Read the comment docs.

[Lee-W]

@woile I'll say this is an ok temporary solution for additional arguments. I think we'll eventually use something like --, but I'm good with this. What do you think?

[woile]

I like it and I'm fine with it, only thing missing is some docs 👍🏻

[woile]

You got it! 🎉