Disable process.exit calls using .exitProcess(false)

I'm writing a CLI that can also be `require()`'d as a node module. I also handle errors generically thru a callback. 
The `process.exit` calls were proving problematic, so here's how I solved it.  
Includes:
-`README.md` entry
-Test Coverage

Let me know if there's any semantic changes you'd like - I've tried to follow your conventions as best possible. 
Also - feel free to toss this if it's something you'd rather not have in yargs core :-)