Make core plugins that depend on colors fall back to global color palette if not specified explicitly

[adamwathan]

PRing this against config-restructuring to make the diff cleaner for now, but ultimately will be targeting the next branch once config-restructuring is merged.

This PR makes it so that changes to the colors object in the theme section of your config file are automatically inherited by the backgroundColors, borderColors, and textColors core plugins.

This means that instead of having to do this:

module.exports = {
  theme: {
    colors: {
      ...defaultTheme.colors,
      customColor: '#bada55',
    },
    backgroundColors: {
      ...defaultTheme.colors,
      customColor: '#bada55',
    },
    borderColors: {
      ...defaultTheme.colors,
      customColor: '#bada55',
    },
    textColors: {
      ...defaultTheme.colors,
      customColor: '#bada55',
    },
  },
}

...or this:

const colors = {
  ...defaultTheme.colors,
  customColor: '#bada55',
}

module.exports = {
  theme: {
    colors: colors,
    backgroundColors: colors,
    borderColors: colors,
    textColors: colors,
  },
}

...you can just do this:

module.exports = {
  theme: {
    colors: {
      ...defaultTheme.colors,
      customColor: '#bada55',
    },
  },
}

I still need to figure out a solution for the borderColors.default thing, because to make this PR work properly I also need to delete the backgroundColors, borderColors, and textColors sections from the default theme. Won't merge until I have that figured out.

[adamwathan]

Closing in favor of #645.