fix: Write errors to stderr

[Morten1337]

This allows you to redirect stderr and stdout separately.

This should not change the behavior of the usage example, so I did not update the README file.

Here's an example on how to write error messages to the console, and the swagger output to a file:

swagger-inline "./*.js" --base 'swaggerBase.json' 2>&1 > api.json

[erunion]

Published to v3.2.3, thanks!