Add Known Folders support on Windows

[adrg]

• Add Known Folders support on Windows
• Internal package refactor
• Improve test coverage

Addresses #24.

[codecov]

Codecov Report

Merging #27 (2928c45) into master (addede4) will increase coverage by 3.55%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           master       #27      +/-   ##
===========================================
+ Coverage   96.44%   100.00%   +3.55%     
===========================================
  Files           8         8              
  Lines         281       381     +100     
===========================================
+ Hits          271       381     +110     
+ Misses         10         0      -10     

Impacted Files	Coverage Δ
base_dirs.go	100.00% <100.00%> (ø)
internal/pathutil/pathutil.go	100.00% <100.00%> (ø)
internal/pathutil/pathutil_unix.go	100.00% <100.00%> (ø)
internal/pathutil/pathutil_windows.go	100.00% <100.00%> (ø)
paths_darwin.go	100.00% <100.00%> (ø)
paths_unix.go	100.00% <100.00%> (ø)
paths_windows.go	100.00% <100.00%> (ø)
xdg.go	100.00% <100.00%> (ø)

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