- when used on the CLI, lib/cli.js was writing the output of log
statements to STDOUT; this caused consumers of the CLI who wanted to
pipe the processed output (css) to other unix utilities to end up with
invalid CSS due to the log messages appearing at the start and end of
the files
- this commit fixes this by replacing `console.log` with `console.warn`
and `console.error`, which both write output to STDERR
