New commandline flag to not check/update the configured files

[balrok]

Use-cases:

• just retrieving the new version without modifying/checking the files
• updating only a particular file with e.g., bump2version --no-configured-files major my-file.txt

With the -h text I'm not sure: I'm open for suggestions :-)

[balrok]

@c4urself can you merge this PR too? this is the last feature to avoid having a custom-built bump2version in our pipelines

Right now I need this for this: bump2version major --allow-dirty --no-configured-files --new-version=$newVersion --search='## Unreleased' --replace='## {new_version} ({now:%d.%m.%Y})' '${changelogFile}' || true"

since changelog and versioning are two concerns in the code and changelog-bumping should only happen on minor-version increments, I need this flag

[ekohl]

I think sufficient time has passed for other reviewers and LGTM.

[c4urself]

thanks!