extra_css files incorrectly reported as unused

[DetachHead]

if the extra_css property is defined in mkdocs.yml, the files are automatically included in the docs, but this plugin treats them as unused:

# mkdocs.yml
extra_css:
  - stylesheets/extra.css

WARNING -  The following files exist in the docs directory, but may be unused:
  - stylesheets/extra.css
Aborted with 1 warnings in strict mode!

i can just add it to excluded_files but i think it should be able to handle this automatically