Use process.exitCode instead of process.exit()

[blicksky]

This pull request replaces calls to process.exit() with assignments to process.exitCode. This is necessary because calling process.exit() forcibly terminates the process without waiting for streams, including stdout and stderr, to be flushed.

From https://nodejs.org/api/process.html#process_process_exit_code:

It is important to note that calling process.exit() will force the process to exit as quickly as possible even if there are still asynchronous operations pending that have not yet completed fully, including I/O operations to process.stdout and process.stderr.

This caused me problems when attempting to spawn nsp as a child process and read the result of the JSON reporter from stdout. The output was always truncated at 8192, and some projects produced output larger than that.

See nodejs/node#12921.

[blicksky]

The tests pass on my machine. It seems that the CI failure is related to finding a .nsprc file in a home directory outside of the project repository, which is out of my control as far as I can tell.

[blicksky]

Fixes #248

[KevinHock]

This should be merged :D Thank you @blicksky for making this.

[KevinHock]

Hi @nlf @wraithgar @evilpacket, can you please merge this PR when you get a chance?

[nlf]

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