Replace bonus-show-start with bonus-style in task

[rabuu]

Describe your changes

• Remove bonus-show-star option.
• Add bonus-style option with improved default value.
• Fix (unrelated) comment issue on score-box.

Note: While bonus tasks are now rendered as "Task X: Name (Bonus)", the star stays the bonus mark in the score box for brevity.

Related issues and/or PRs

Resolves #57.

Checklist

• The test suite passes
• The code is formatted
• The code is readable and well-documented
• Every word that can be in the output is translatable
• All new features are reflected in a test or example
• All new examples have a generated associated test
• Changes to the public API are documented in the CHANGELOG
• All related PRs and issues are linked
• An appropriate label is chosen

[leanderhert]

Nice!
The new behavior is very reasonable and nice implemented.
Since the conflict is just regarding the CHANGELOG, I have nothing to add. :)