Allow LB handling choice, defaults to GFM style

- Less technical users tend to find the default Markdown linebreak
handling confusing. Which was the reason for the change in the first
place but this allows for one to choose which handling works best for
their use case
- Defaults to Github-style-line-breaks for those who are already
accustomed
- Probably going forward all customizations could be in the
extensions/config.php ... figured that was elegant-ish/the best place to
put it