fix: support .attrs method

[gregberge]

Closes #7

[AlxGolubev]

argh, I spent half an hour to find a place where I can put a code to collaborate. Good job, highly appreciated!

[codecov]

Codecov Report

Merging #8 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master       #8      +/-   ##
==========================================
+ Coverage   98.86%   98.87%   +<.01%     
==========================================
  Files          21       21              
  Lines         353      356       +3     
  Branches       54       54              
==========================================
+ Hits          349      352       +3     
  Misses          3        3              
  Partials        1        1

Impacted Files	Coverage Δ
packages/styled-components/src/styled.js	100% <100%> (ø)	⬆️

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