Not rewriting config file when bumping

[balrok]

Since pr #90 stalled, I fixed the missing tests. By doing that, it also uncovered some regressions.

1. when current_version did not exist in .bumpversion.cfg, it will not be created:

• when retrieving the version from the git tag
• when retrieving the version via --current_version flag

2. when .bumpversion.cfg contains a "new_version" field, previously this
   field was removed, but with this change it is not possible anymore

Solutions:
Not staying backwards compatible, because:

1. is a relatively uncommon, once this case happens the file has the "current_version" written afterwards, so it might not affect many
2. even if there is a use-case for this, it can be easily migrated to print an error when new_version is found in the config and recommend "--new_version="

Staying backwards compatible, but being less intuitive:
Additionally, write the file whenever case 1 or 2 is true. This can be implemented by letting _load_configuration return a boolean.

This would solve:

• Not incrementing version on all lines inside setup.cfg #87 Add support for metadata.version as canonical version #62
• Strip trailing white spaces before saving the config #161
  And would simplify the implementation for pyproject.toml (PEP 518) support #42, since only a toml-parser but not printer is required

[balrok]

I found a third option, which avoids breaking backwards comaptibility. Closing it now.