feat(args): add --no-exec flag for skipping command execution

[orhun]

Description

Adds a new flag for skipping command execution:

git cliff --no-exec

Motivation and Context

closes #333

How Has This Been Tested?

Test fixtures

Types of Changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (no code change)
• Refactor (refactoring production code)
• Other

Checklist:

• My code follows the code style of this project.
• I have updated the documentation accordingly.
• I have formatted the code with rustfmt.
• I checked the lints with clippy.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov-commenter]

Codecov Report

Attention: 7 lines in your changes are missing coverage. Please review.

Comparison is base (722efd6) 41.69% compared to head (d16d88e) 41.40%.

Files	Patch %	Lines
git-cliff/src/lib.rs	0.00%	7 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #458      +/-   ##
==========================================
- Coverage   41.69%   41.40%   -0.28%     
==========================================
  Files          15       15              
  Lines        1022     1029       +7     
==========================================
  Hits          426      426              
- Misses        596      603       +7     

Flag	Coverage Δ
unit-tests	41.40% <0.00%> (-0.28%)	⬇️

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