Write errors to stderr

[Morten1337]

In src/cli.js the program errors are written out with console.log.

Consider using console.error instead so that stderr and stdout can be redirected separately.

Like in my case I only want the the swagger output written to a file, and if there is a parsing error, I'd like to see that in the shell.

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

    .catch(err => {
-      console.log('An error occured:');
-      console.log(err);
+      console.log('An error occured:');
+     console.log(err);
      process.exit(-1);
    });

[erunion]

@Morten1337 If you're up for submitting a pull request we'll happily pull it in.

[erunion]

Done in #179