Initial folder support

This PR starts to address https://github.com/SlexAxton/css-colorguard/issues/7, a few questions:

- The input currently concatenates all the CSS files together and reports line numbers that aren't useful anymore past the first file). What's the ideal output format in this case? I see two ways of approaching this: 
  - handle each CSS file individually (file name keys to current output blob)
  - handle them all together and add support in the output for mapping line number + file name (`line: [1 (foo.css), 1 (bar.css)]`)
- Recursively look in directory for CSS files or top level of folder?