Refactor config autoload

[cap-Bernardito]

This PR contains a:

• bugfix
• new feature
• code refactor
• test update
• typo fix
• metadata update

Motivation / Use-Case

• Moved the config autoloading logic from the postcss-load-config package inside the loader
• Add config option types boolean and string

Breaking Changes

No

Additional Info

No

[codecov]

Codecov Report

Merging #440 into master will increase coverage by 2.93%.
The diff coverage is 96.90%.

@@            Coverage Diff             @@
##           master     #440      +/-   ##
==========================================
+ Coverage   92.74%   95.67%   +2.93%     
==========================================
  Files           5        6       +1     
  Lines         124      208      +84     
  Branches       37       64      +27     
==========================================
+ Hits          115      199      +84     
  Misses          9        9              

Impacted Files	Coverage Δ
src/utils.js	96.70% <96.70%> (ø)
src/index.js	92.50% <100.00%> (+2.84%)	⬆️

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