Allow accessing deep paths with function values via theme helper

[CvX]

Example:

theme: {
  spacing: {
    '0': '0',
  },
  width: theme => ({
    ...theme('spacing'),
    '1/3': '33.33333%',
  }),
  minWidth: theme => ({
    // This didn't work because `width` is a function.
    // It had to be written as `theme('width')['1/3']`
    '1/3': theme('width.1/3'),
  }),
}

[MichaelDeBoey]

This one could resolve #869

[CvX]

Ha, if I hadn't missed that issue, I'd save some time debugging. 😉

[adamwathan]

This is amazing, thanks! 🙌🏻