Support defining presets as functions

[RobinMalfait]

Description by @adamwathan

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This PR adds support for defining presets as functions in case users need to accept options in their presets. We don't actually provide anything to these functions, we just notice if you've given us a function and invoke it with no arguments on your behalf. This lets your preset be used like this:

module.exports = {
  // ...
  presets: [
    // Require it with no options
    require('@acmecorp/tailwind-base'),

    // Invoke it and pass options to your own options API
    require('@acmecorp/tailwind-base')({
      colorPalette: 'legacy',
    })
  ]
}

We need this internally for the Tailwind UI plugin which we're currently converting to a preset for Tailwind 2.0.

[codecov-io]

Codecov Report

Merging #2680 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2680   +/-   ##
=======================================
  Coverage   92.99%   92.99%           
=======================================
  Files         172      172           
  Lines        1727     1727           
  Branches      304      305    +1     
=======================================
  Hits         1606     1606           
  Misses        104      104           
  Partials       17       17           

Impacted Files	Coverage Δ
src/util/getAllConfigs.js	75.00% <100.00%> (ø)

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