Add todo(signal) option

[rabuu]

Describe your changes

This adds the signal option for the todo function to make it possible to disable signalling the TODO warning to the context. This is useful when you want to create a TODO box locally, but don't want it to propagate to the task or global heading.

Related issues and/or PRs

Resolves #83.

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