Ensure that all lines of underscores have at least two preceding newlines

Some users may reply directly above a line of underscores, because the gap between lines can be a little misleading, e.g.

```
Outlook with a reply directly above line
________________________________________
From: CRM Comments [crm-comment@example.com]
Sent: Friday, 23 March 2012 5:08 p.m.
...
```

Unfortunately, the parser cannot split fragments without empty lines between them.

This pull request solves that problem by ensuring that all underscore lines are preceded by at least two newline characters. (I can't think of any situation where that would cause a problem.)

The previous example will be 'pre-processed' into:

```
Outlook with a reply directly above line

________________________________________
From: CRM Comments [crm-comment@example.com]
Sent: Friday, 23 March 2012 5:08 p.m.
...
```

But if *this* ^ is run through the parser, it will not be affected.