[FEATURE]: Allow LayoutProps to be used directly

[hirokazutei]

Currently, the layout prop is passed in like:

<Inset all={16} layout={{display: 'flex'}} />

However, it is also possible to have designed it so that we can:

<Inset all={16} display='flex' />

It is not a good idea to allow both since there will be more than one way to layout.
The second option was chosen since #134 is not yet available and I wanted to keep to focus of Inset to still be about spacing child element(s) from the elements that surround it. (And using ES6 spread directly into Inset tends to break flow... 😞)

But in the future, given #134's ability to choose strictness, the props might not be too overwhelming and it might be good to give users the choice to have Inset take props like the 2nd example.