Use emotion to render css

[DecampsRenan]

The idea is to manage theme and css via emotion; this could enable some nice features like:

• as prop
• Native support of hardcoded css values (right now, only theme values are accepted => we can't work with dynamic colors sent by the api or defined by the user for example)
• Alias support (not sure), so using h or height is the same

Usage:

<Box as={Link} color="red.200" />

Maybe using something like emotion could help (see https://emotion.sh/docs/styled#as-prop)