Support for nested tabs?

[greatislander]

Hi @zachleat! I'm wondering if you'd be open to a PR to add support for nested tabs for this component? The changes needed are fairly minor. Here's a codepen (JavaScript changes are on lines 12, 13, and 60):

https://codepen.io/greatislander/pen/GRNxgZY

Let me know— if that's outside of your intended scope, no worries!

[zachleat]

Yeah absolutely! I’d love a PR!