Check if there are updates to the git repository

Hello

Would you be interested in this feature?

In my use case I don't use npm, I have a private git repository that other people can clone. This repo contains some scripts and I want to notify other people when a new commit was pushed to this repo.

It works exactly the same as npm but the configuration is

```js
updateNotifier({
	packageName: pkg.name,
	remote: "origin"
}).notify();
```

The code will then fetch changes from the `remote` and check if the current branch is behind the same branch on the configured remote.

This PR is mostly a proof of concept, if you like the idea I'll create a separate module for `latestGit` and add some unit tests.

Cheers