GFM: Line break using a backslash

The GFM spec specifies that to make a line break, [a backslash can be used](https://github.github.com/gfm/#hard-line-breaks) so it is more visible. It also better because some editors automatically remove trailing whitespace.

The `GithubMarkdown` class [doesn't support those](http://markdown.cebe.cc/try?flavor=gfm&text=test%5C%0D%0Atest). Only double-space breaks are allowed. It would be really nice if that feature would be added.

I tried doing this myself, but `renderText` receives line text and the backslash as separate blocks, so I kinda failed there. Also, what would be the preferred way to do this? Is changing the escape code parser necessary?