Update flex component names

[broccolini]

Could we rename FlexContainer to Flex and FlexItem to Flex.Item?

It seems like we're all on board with the . right? And I think Flex works well as the base component name.

[shawnbot]

Yes! We discussed doing this in #166, and it sounds like we've made our decision. Let's close that and open other issues like this for specific components.

[shawnbot]

Done in #315! 🎉