Support multiple attributes

[vkostyukov]

This introduces multipartAttributes and multipartAttributesNel.

[codecov-io]

Codecov Report

Merging #872 into master will increase coverage by 0.45%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #872      +/-   ##
=========================================
+ Coverage   81.45%   81.9%   +0.45%     
=========================================
  Files          51      51              
  Lines         744     746       +2     
  Branches       26      34       +8     
=========================================
+ Hits          606     611       +5     
+ Misses        138     135       -3

Impacted Files	Coverage Δ
...e/src/main/scala/io/finch/endpoint/multipart.scala	76.66% <100%> (-5.48%)	⬇️
core/src/main/scala/io/finch/Input.scala	100% <0%> (+12.5%)	⬆️

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