Exit from Node without terminating process

**Is your feature request related to a problem? Please describe.**
The two ways to stop ffmpeg from running in Node are either `ffmpeg.exit()` or `process.exit()`, both of which immediately terminate the Node process and prevent further work from being done.

**Describe the solution you'd like**
Have `ffmpeg.exit()` terminate the long-running Node process without terminating the overall process.

**Describe alternatives you've considered**
Passing in an optional error code into `exit`. While needed so it doesn't always throw an error, in and of itself it won't resolve the ability for this to be able to be integrated into other tools.