Add an option to ignore files not tracked by Git when checking status

[BehindTheMath]

Currently, the only options are the default, which checks all files not included in .gitignore, and --skip-status which doesn't check anything. It would be nice to have an option to only check files that are tracked by Git, but ignore anything else.

[MoOx]

PR welcome. That said, I won't that to be the default behavior as many people like this feature. It's handy to push to npm some generated files (eg es6 to es5 code in ignored dist (or lib) folder.