Return command output in Remote\Connection::run

Currently the only way to get the actual output of a `SSH::run('ls')` per example is to do this :

```php
$output = '';
SSH::run('ls', function ($line) use (&$output) {
  $output .= $line;
});

dd($output);
```

I thought this was a lot of going around for something the `run` command can just return, so that's what this PR does :

```php
$output = SSH::run('ls');
dd($output);
```