Let root_dir work when running on directories

[gjtorikian]

Should close #587

[codecov-io]

Codecov Report

Merging #594 (26c97fa) into main (5badc4e) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #594      +/-   ##
==========================================
+ Coverage   98.01%   98.06%   +0.04%     
==========================================
  Files          31       31              
  Lines        2063     2064       +1     
==========================================
+ Hits         2022     2024       +2     
+ Misses         41       40       -1     

Impacted Files	Coverage Δ
lib/html-proofer/element.rb	96.92% <100.00%> (+0.68%)	⬆️
lib/html-proofer/runner.rb	99.29% <100.00%> (ø)
spec/html-proofer/command_spec.rb	100.00% <100.00%> (ø)

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