Add support for newlines in -a="" arguments

Previously, if you did this in bash:
```bash
$ node program.js --foo="hey
  bar"
```
You'd end up with args.foo === 'hey'
Now you get the correct args.foo === "hey\nbar"

Note: the below WAS workign and continues to work (no = )
```bash
$ node program.js --foo "hey
  bar"
```