Support direction, justifyContent, and alignItems props on EuiFlexItem

[cjcenizal]

This element currently has display: flex on it, which can make it challenging to position content inside of it. The only way to do this is with a custom class (example]. A simple fix would be to support these props the same way EuiFlexGroup does.

[cjcenizal]

CC @alisonelizabeth

[snide]

As mentioned here #3069 (comment) we've decided to close out this issue. We're a little worried about the prop jumble of these components and think it's easier to simply suggest using a wrapping element around your content. Given the lack of conversation on this issue, we assume that method has worked well enough for folks.