add support for align-items: baseline

[stefanosambruna]

Add support for align-items: baseline to all the layout components that support this property.

Why do I think it's important?
I would like to have on the same line two texts with different sizes (line-height, font-size etc.). For instance:

<Inline space={24} alignY="bottom">
   <Display size="medium">OOOOO</Display>
   <Headline size="medium">OOOOO</Headline>
</Inline>

which, I guess, corresponds to align-items: bottom and it results in:

As you can see from the image above the two texts do not look aligned. If I use baseline instead, I get something like this:

[gabro]

Done in #446. It will be released in 0.15.2