Lazily evaluate values in the extend section

[CvX]

In a way this combines PRs #655 and #774. I've stumbled upon this use case when trying out the 1.0 beta:

module.exports = {
  theme: {
    colors: {
      red: 'red',
      green: 'green',
      blue: 'blue',
    },
    extend: {
      borderColor: theme => ({
        default: theme('colors.red'),
      }),
    },
  },
};

With 1.0.0-beta.2 you have to choose between using the extend section and having lazy evaluated value.

[adamwathan]

Ah good catch, thanks!