feat: add custom validation

[benediktziegler]

Description

This PR adds functionality to customise the commit message validation and to format the InvalidCommitMessageError to give better/more detailed feedback to the user.

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

The developer of a custom commitizen class can override the validate_commit_message and format_error_message methods to perform more complex commit message format checks then just a regex match and give more detailed feedback on failure.

Steps to Test This Pull Request

Run the the test_check_command_with_custom_validator_succeed and test_check_command_with_custom_validator_fail tests in test_check_command.py.

Additional context

This PR implements and fixes the comments from #648.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.57%. Comparing base (120d514) to head (8dfe7b6).
Report is 429 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1236      +/-   ##
==========================================
+ Coverage   97.33%   97.57%   +0.24%     
==========================================
  Files          42       55      +13     
  Lines        2104     2520     +416     
==========================================
+ Hits         2048     2459     +411     
- Misses         56       61       +5     

Flag	Coverage Δ
unittests	97.57% <100.00%> (+0.24%)	⬆️

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

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[benediktziegler]

@Lee-W I was wondering if you had any input to this PR?

[Lee-W]

I'll be mostly out till at least mid-Oct, will try to check in depth after that. Thanks!