Nested flex

[giulioz]

Closes #10
and also regards #7 if some optimizations are considered

This fixes some things to make nested Flexbox work again properly.
README is still a WIP.

This changes nested flex semantic:

OLD

<Flex flexDirection="row" flexWrap="wrap" size={[50, 0, 0]}>
  <Box>
    <Flex flexDirection="column" flexWrap="no-wrap">
      <Box>
        <mesh geometry={sphere} />
      </Box>
      <Box>
        <mesh geometry={box} />
      </Box>
    </Flex>
  </Box>
</Flex>

NEW

<Flex flexDirection="row" flexWrap="wrap" size={[50, 0, 0]}>
  <Box flexDirection="column" flexWrap="no-wrap">
    <Box>
      <mesh geometry={sphere} />
    </Box>
    <Box>
      <mesh geometry={box} />
    </Box>
  </Box>
</Flex>

[giulioz]

I think that passing flexProps can be a bit slow BTW

[giulioz]

centerAnchor is a new prop that you have to set if your children has position centered.
While a inside another is relative to the upper left corner, all THREE standard shapes are referred to the center.

[giulioz]

This way we can force width and height directly, ignoring the content bounding box.
This could be optimized preventing the bounding box calculation if props are present.