feat: add attrs option

[dutzi]

This PR contains a:

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

Motivation / Use-Case

We're using css-vars-ponyfill for IE11, we are a widget embedded in a website, we want css-vars-ponyfill to run only on CSS generated by us. We need a way to distinguish out CSS tags from others.

[jsf-clabot]

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[codecov]

Codecov Report

Merging #445 into master will increase coverage by 0.14%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #445      +/-   ##
==========================================
+ Coverage   88.32%   88.46%   +0.14%     
==========================================
  Files           4        4              
  Lines         411      416       +5     
  Branches       86       88       +2     
==========================================
+ Hits          363      368       +5     
  Misses         46       46              
  Partials        2        2              

Impacted Files	Coverage Δ
src/index.js	88.28% <100.00%> (+0.26%)	⬆️

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[dutzi]

Hey, I don't understand what Lint errors it's complaining about, can anyone help?
It only says:

Bash exited with code '1'.

Also, can't understand why tests are failing? Locally they run smoothly.

[dutzi]

Any update?