UnderlineNav follow up items

[emplums]

🐛 Bugs

• Underline is red-orange when Kit is first loaded, but once you navigate to other tabs, it turns grey.

🚀 Enhancements

• Spread children's props back onto cloned child
• Move rendersClass into utility file
• Consider whether or not it's ok to have a circular dependency between UnderlineNav and UnderlineNavLink

[shawnbot]

All of the enhancements listed above are addressed in #134.