[Console] Added support for labels to ProgressHelper

Allows the addition of a label to the progress helper.  For long(er) running tasks it could be nice to provide some additional information on what is currently happening in textual form.

```
$progress = $this->getHelperSet()->get('progress');
$progress->addLabel('Starting download', $progress::LABEL_BOTTOM, $progress::LABEL_CENTER);
$progress->start($output, $end);
$progress->display();
for ($i = 0; $i < $end; $i++) {
    $x = $i + 1;
    $progress->updateLabel('Downloading ' . $x);
    $progress->advance();
    sleep(1);
}
$progress->finish();
```

The label can be position above or under the progress bar, and left/center/right aligned.  To allow for positioning over the progress bar, I had to move the line length padding code from overwrite() to display().

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        |