Use exitCode instead of exit()

[qfox]

To provide a better user experience we should not force event loop to stop since some library code can require additional things to do.

In other words we should set exitCode instead of calling exit() method. https://github.com/veged/coa/blob/fb1080e151e55da00c8c68e88a32b5b8a723549d/lib/cmd.js#L197