Allow linebreaks in source (classic Markdown style)

Hi, 

This is a small feature : the `softLinebreaks` config option makes Wikirenderer to treat linebreaks in blocks as normal formatting. So it doesn't end in a `<br />`.

The default configuration (`false`) stills behaves as normal Skriv, and `true` enables this feature. I've also added a symbol to create `br` tags when this mode is enabled : `-/-`. 

Thanks for any comment.

Lud