feat(args): set CHANGELOG.md as default missing value for output option

[sh-cho]

Closes #352

Description

$ git cliff # outputs to stdout

$ git cliff -o # outputs to CHANGELOG.md

$ git cliff -o OTHER_FILE.md # outputs to another file

If -o is given and output path is omitted, output to CHANGELOG.md

For output argument, I set num_args to 0..=1 and default_missing_value to CHANGELOG.md.

Motivation and Context

• Set default output as CHANGELOG.md #352

How Has This Been Tested?

I builded locally and tested with commands like below,

$ cargo build --release
$ target/release/git-cliff                   # printed via console
$ target/release/git-cliff -o                # CHANGELOG.md is generated
$ target/release/git-cliff -o -c cliff.toml  # CHANGELOG.md is generated
$ target/release/git-cliff -o ABC.md         # ABC.md is generated

Screenshots / Logs (if applicable)

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.

Well.. I started to study Rust for the first time a few hours ago 😂

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (8edec7f) 44.43% compared to head (64219b0) 44.43%.

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Additional details and impacted files

@@           Coverage Diff           @@
##             main     #354   +/-   ##
=======================================
  Coverage   44.43%   44.43%           
=======================================
  Files          13       13           
  Lines         628      628           
=======================================
  Hits          279      279           
  Misses        349      349           

Flag	Coverage Δ
unit-tests	44.43% <ø> (ø)

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[orhun]

Lovely, thanks for your contribution! 💖

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