Add skip option for (sub)tasks

[rabuu]

Describe your changes

Add option skip for task and subtask. It simply steps the counter by the given amount.

Related issues and/or PRs

Resolves #54.

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]

Cool!
I have nothing to add.
(I think it's reflected in exampletask-config.typ ;) )