Add timestamp attribute

[mpkorstanje]

🤔 What's changed?

While the timestamp attribute is not part of the JUnit or Surefire XSD in practice it seems to be a common enough property[1] that we can add it without expecting any of the popular tools to break.

Closes: #44

1. https://github.com/testmoapp/junitxml

🏷️ What kind of change is this?

• ⚡ New feature (non-breaking change which adds new behaviour)

📋 Checklist:

• I agree to respect and uphold the Cucumber Community Code of Conduct
• I've changed the behaviour of the code
  • I have added/updated tests to cover my changes.
• My change requires a change to the documentation.
  • I have updated the documentation accordingly.
• Users should know about my change
  • I have added an entry to the "Unreleased" section of the CHANGELOG, linking to this pull request.

[davidjgoss]

JS implementation done!

[mpkorstanje]

Great then we just have to update the docs.