feat(args): support initialization with built-in templates

[orhun]

Motivation

Now we embed the files under examples/ into the binary and support the following use cases:

# creates cliff.toml with keepachangelog template
git cliff --init keepachangelog

# generates a changelog in keepachangelog format
git cliff --config keepachangelog

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: 21 lines in your changes are missing coverage. Please review.

Comparison is base (0945fa8) 43.76% compared to head (d104c20) 42.65%.

Files	Patch %	Lines
git-cliff/src/lib.rs	0.00%	12 Missing ⚠️
git-cliff-core/src/embed.rs	0.00%	9 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #370      +/-   ##
==========================================
- Coverage   43.76%   42.65%   -1.11%     
==========================================
  Files          13       13              
  Lines         649      666      +17     
==========================================
  Hits          284      284              
- Misses        365      382      +17     

Flag	Coverage Δ
unit-tests	42.65% <0.00%> (-1.11%)	⬇️

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[liuyinz]

@orhun Brilliant work !
how about more options related to built-in templates? e.g.

# add directory which stores user-defined templates, 
git cliff --templates-dir <PATH> (ENV: GIT_CLIFF_TEMPLATES_DIR=)

# print all available templates, both built-in and user-defined templates, which is needed for providing completion in emacs.
 git cliff --list-templates

these features are implemented in my emacs plugin for now and glad for more features are supported natively.

[orhun]

@liuyinz thank you! Those are great suggestions.

Would you be potentially interested in contributing this? Either way, I would appreciate a tracking issue to implement them in the future.

[liuyinz]

@orhun I know nothing about rust.. so waiting :>

[orhun]

Can you at least create an issue so that I won't forget about this feature? :D

[liuyinz]

#412 created. My job: done!