Add LINEBREAK options to switch between linebreak behaviours

[mrombout]

Twine storyformats do not consistently follow the same behaviour to handle newlines. This causes an issue with multiple passage links seperated by a single linebreak (so no white line in between).

See #40 for the original bug report regarding this issue.
See #46 for more information on why Spiner needs this option.

Depends on #49 to provide the options mechanism.

[mrombout]

Might want to hold off on this one since I am considering only supported one storyformat for now. And even if I would, I think a better choice would be to update the architecture to allow for interchangable storyformats that match the behaviour of the respective story format.