Core: Allow custom postcss config

[thollander]

• Search for the nearest postcss config
• Maintain previous defaults if no postcss config

Issue: #2455
Old PR: #4411

What I did

• Add find-up to dependencies to search for the nearest postcss.config.js
• Use the nearest postcss config (.postcssrc.json, postcss.config.js...) to determine postcss-loader options if present, maintain previous defaults if not.

How to test

Run the example storybook/examples/postcss-kitchen-sink.
The h1 should have the color: #639; instead of color: rebeccapurple;.

• Is this testable with Jest or Chromatic screenshots?
  No
• Does this need a new example in the kitchen sink apps?
  I created one, not sure if it was mandatory.
• Does this need an update to the documentation?
  Yes, I added a documentation in /custom-postcss-config

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[shilman]

Thanks for putting this together @thollander !

A few thoughts:

• Would it be possible to add the example to an existing examples/ project rather than creating a new one? I don't want to increase our build times if possible.
• Right now postcss support is implemented in core, but I'm wondering if we can move it into a preset? Any chance you might be able to take this on? I'd like to move as much as possible out of core to make the default storybook experience lean and mean. This would be a breaking change, so the first step would be to move this into a preset and always include that preset. In a subsequent release (6.0?) we'll make it easy for users to opt-in to those presets

[shilman]

Ace 🎯

[thollander]

Would it be possible to add the example to an existing examples/ project rather than creating a new one? I don't want to increase our build times if possible.

Yes for sure, I did it. You’re totally right it is not necessary to increase the build time just for this. 

Right now postcss support is implemented in core, but I'm wondering if we can move it into a preset? Any chance you might be able to take this on? I'd like to move as much as possible out of core to make the default storybook experience lean and mean. This would be a breaking change, so the first step would be to move this into a preset and always include that preset. In a subsequent release (6.0?) we'll make it easy for users to opt-in to those presets

I get your point but not sure how to implement it yet. I’ll give it a try. 
If you’re ok with theses changes you can still merge this PR.
If I succeed to manage as a preset I would open another PR.