Add more advanced usage examples to render functions guide

[chrisvfritz]

Including:

• Bubbling up events from child components and wrapping event handlers from the parent
• Modifying the data object of slot components with lodash's merge on VNodes

[sdras]

Would this be a good candidate for cookbook or should it stay with the guide? I feel like we definitely should include a render function example in the cookbook

[chrisvfritz]

I think it's a good candidate for the cookbook. 🙂