Add flexbox support

[sornian]

https://github.com/facebook/yoga

Yoga is a cross-platform layout engine which implements Flexbox. 

[SpartanJ]

I don't think yoga flexbox implementation would go well with the engine. The Flexbox layout needs to be implemented natively to be performant, it isn't that hard to add a Flexbox layout, I just didn't have the opportunity yet (but it's planned). There are some reference implementations that are very small and can be taken as a reference in the future, for example:
https://github.com/randrew/layout and https://github.com/Sleen/FlexLayout
This is an interesting project for a collaborator, anyone is more than invited to look into it if interested, I can provide support for the implementation.