Use numbers instead of bullets for ordered lists

Currently ordered lists are converted into unordered lists in text; using an asterisk instead of numbers.  Would be great to use numbers for these cases.


## Example
```php
(new \Html2Text\Html2Text('<ol><li>Item 1</li><li>Item 2</li></ol>'))->getText()
```
### Result
```
* Item 1
* Item 2
```

### Expected
```
1. Item
2. Item
```