Fully support Inspec attributes files.

[fatmcgav]

Change Attrs to be a string. Add validation for filepath. Pass through to Inspec CLI if set. Add test coverage.

[jrbeilke]

This looks good, but why the switch to a single string for attrs when inspec supports passing multiple attributes files at a time?

[fatmcgav]

@jrbeilke Good question, and yeh, looks like it will support multiple files with a single flag: https://github.com/inspec/inspec/blob/6f8c0245c8b980fde477948c97d3e95e095ada82/lib/inspec/base_cli.rb#L104-L105

I'll re-work to loop over attrs...

[fatmcgav]

@jrbeilke Updated in b27552a :)

[fatmcgav]

Hey @jrbeilke - any chance of getting this one merged and released? :)