[Plugin] Extending styles

[robinweser]

Environment: Both
Type: Feature

---

Description

Would be great to have a plugin which allows (conditional) style object extension.

Proposal

We could use the extend key to specify styles used to extend.

Code Example:

// basic
const baseRules = { backgroundColor: 'red' }
const rule = props => ({
  color: 'blue',
  // might support multiple objects using arrays
  extend: baseRules
})

// conditional
const rule = props => ({
  color: 'blue',
  // could also support multiple using arrays
  extend: {
    condition: props.bg === true,
    style: {
     backgroundColor: 'red' 
   }
  }
})