Use postcss-nested and postcss-safe-parser instead of styled-components fork and pass filename to postcss

[emmatown]

What:

Use postcss-nested and postcss-safe-parser instead of styled-components fork and pass the filename to postcss with the from option

I also moved the object parser into the src since we need to change it for #189

Why:

#188

How:

Checklist:

• Documentation
• Tests
• Code complete

I'm pretty sure I'm passing the filename to postcss incorrectly since I don't think the .browserslistrc file is being read

Closes #188

[codecov-io]

Codecov Report

Merging #192 into master will decrease coverage by 0.72%.
The diff coverage is 77.27%.

@@            Coverage Diff            @@
##           master    #192      +/-   ##
=========================================
- Coverage   89.13%   88.4%   -0.73%     
=========================================
  Files          22      23       +1     
  Lines         847     880      +33     
  Branches      219     231      +12     
=========================================
+ Hits          755     778      +23     
- Misses         70      77       +7     
- Partials       22      25       +3

Impacted Files	Coverage Δ
src/parser.js	100% <100%> (ø)	⬆️
src/babel.js	98% <100%> (+0.02%)	⬆️
src/obj-parse.js	69.69% <69.69%> (ø)
src/react/index.js	100% <0%> (ø)	⬆️

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[ai]

@mitchellhamilton great work! Thanks. I will announce it in PostCSS twitter after the release.

@tkh44 @mitchellhamilton also you can clean up code a little:

function stringifyCSSRoot (root) {
  return root.nodes.map((node, i) => {
-    let str = ''
-    stringify(node, x => {
-      str += x
-    })
-    return str
+   node.toString()
  })
}

In this case you even don’t need to import postcss/lib/stringify.