csslib: add in, out option for cli, remove alot unused imports, update README

* provide a more usefull example in README
* remove **alot** unused imports
* CLI now has options to specify input/output file path, examples:
    * --in app/myStyles.scss --out app/myStyles.css
    * -o app/myStyles.css
* original behavour is preserved (add input file as last argument, use name as base for output file)