Feature/changelog stdout

[woile]

Description

Include a new flag on bump to send the changelog to stdout.

This allows the incremented changelog data to be captured and sent to a file.

I decided to reuse the changelog with --dry-run when this flag is set, instead of adding a new flag on changelog.
I think this is not expensive because the changelog will only be generated for just a single release/tag.

Checklist

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

A normal bump should happen + changelog to the stdout

Steps to Test This Pull Request

1. Create a new commit.
2. Run cz bump --changelog --changelog-to-stdout > body.md
3. Observe body.md which should contain only the incremented changes

Additional context

Closes #225

[codecov]

Codecov Report

Merging #359 (afc5749) into master (ed06664) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #359      +/-   ##
==========================================
+ Coverage   97.53%   97.56%   +0.02%     
==========================================
  Files          39       39              
  Lines        1340     1356      +16     
==========================================
+ Hits         1307     1323      +16     
  Misses         33       33              

Flag	Coverage Δ
unittests	97.56% <100.00%> (+0.02%)	⬆️

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Impacted Files	Coverage Δ
commitizen/cli.py	97.50% <ø> (ø)
commitizen/__version__.py	100.00% <100.00%> (ø)
commitizen/commands/bump.py	95.19% <100.00%> (+0.74%)	⬆️
commitizen/out.py	100.00% <100.00%> (ø)

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[Lee-W]

Sorry for the late review. I was a bit busy recently. This one looks good to me 😃 Let's merge it