feat(commands/commit): add manual-edit functionality after answering questions

[josix]

Description

Add one argument -e/--edit to commit command, to allow user to continue editing after filling up all the questions.
(Kudos to @MrMino! The pytest-edit project inspired me to come up with this idea.)

Checklist

• Add test cases to all the changes you introduce
• Run ./scripts/format and ./scripts/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

if -e/--edit enabled, the terminal would pop up editor with commit message after finishing the answering.

Steps to Test This Pull Request

1. check out to this branch
2. make a commit with cz c -e

[codecov]

Codecov Report

Attention: Patch coverage is 92.59259% with 2 lines in your changes missing coverage. Please review.

Project coverage is 97.61%. Comparing base (120d514) to head (f73d452).
Report is 461 commits behind head on master.

Files with missing lines	Patch %	Lines
commitizen/commands/commit.py	90.90%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1262      +/-   ##
==========================================
+ Coverage   97.33%   97.61%   +0.27%     
==========================================
  Files          42       55      +13     
  Lines        2104     2602     +498     
==========================================
+ Hits         2048     2540     +492     
- Misses         56       62       +6     

Flag	Coverage Δ
unittests	97.61% <92.59%> (+0.27%)	⬆️

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[Lee-W]

Left one nit, but mostly looks good!