Add remote upstream repository

### Add remote upstream repository to the forked projects after cloning

Could be possible to add the upstream original repository for the forked repositories?

Every time I fork a project on Github I have to manually execute the following command:

```shell
git remote add upstream GIT_UPSTREAM_URL
```

This way I can keep the fork synchronized with the original repo.

I think it could be possible to add an option:

```shell
ghcloneall --include-forks --set-upstream
```

What do you think about it?

The issue names no files or tests. Start by locating ghcloneall's command-line option handling and the flow used to clone forked repositories; done means --set-upstream adds the original GitHub repository as upstream when --include-forks is used.

Written by the indexing model from the issue text.