Add changelog argument options

[janw]

Description

As described in #291, this adds config options for the changelog arguments --incremental and --start-rev to commitizen.

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

• With changelog_incremental = true in the config file, commitizen will only ever attempt incremental changelog updates.
• With changelog_start_rev = <rev> in the config file, commitizen will always update the changelog starting from the given rev.

Steps to Test This Pull Request

changelog_incremental

1. Add changelog_incremental = true to pyproject.toml/.cz.toml
2. add e.g. a custom preamble to the changelog
3. add changes and commit them as feat/fix
4. run cz changelog
5. have changelog updated and the preamble kept intact

changelog_start_rev

1. add changes and commit them as feat/fix
2. add changes and commit them as feat/fix and note rev
3. Add changelog_start_rev = <rev> to pyproject.toml/.cz.toml where is the noted rev from the second commit
4. run cz changelog and see the first commit not being added as an entry to changelog

Additional context

#291

[codecov]

Codecov Report

Merging #294 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #294   +/-   ##
=======================================
  Coverage   96.64%   96.64%           
=======================================
  Files          33       33           
  Lines         923      923           
=======================================
  Hits          892      892           
  Misses         31       31           

Flag	Coverage Δ
unittests	96.64% <100.00%> (ø)

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

Impacted Files	Coverage Δ
commitizen/defaults.py	100.00% <ø> (ø)
commitizen/__version__.py	100.00% <100.00%> (ø)
commitizen/commands/changelog.py	97.29% <100.00%> (ø)

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[woile]

Nice, thanks :D