Add support for `dir-dependency` message type

[bradlc]

This pull request adds support for the dir-dependency message type, detailed in the PostCSS docs.

[codecov[bot]]

Codecov Report

Merging #374 (71aa074) into master (a03c7bf) will decrease coverage by 0.26%. The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master     #374      +/-   ##
==========================================
- Coverage   92.94%   92.68%   -0.27%     
==========================================
  Files          10       10              
  Lines         326      328       +2     
  Branches      115      116       +1     
==========================================
+ Hits          303      304       +1     
- Misses         22       23       +1     
  Partials        1        1              

Impacted Files	Coverage Δ
src/postcss-loader.js	91.17% <50.00%> (-0.83%)	:arrow_down:

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