Add `--clone,-c` option to clone repositories

Fix #7
This only makes sense we if don't want to display the following message if there's no repositories:
```
$output->writeln('<comment>Cloning repositories</comment>');
```

This method, by its name *(cloneRepositories)*, has the responsibility to clone repositories and only that.
I mean, in theory it receives a non-empty array and deal with this.

The check must be done [before the call to this method, or here](https://github.com/henriquemoody/forker/blob/clone/src/kinncj/Forker/Command/Fork.php#L65)