attempt to implement a laxSpacing option

The idea here was to match the LAX_SPACING option from the sundown parser. See [here](https://github.com/vmg/sundown/blob/37728fb2d7137ff7c37d0a474cb827a8d6d846d8/src/markdown.c#L1464-L1484).

Due to the fact that the parsing is *completely* different, I'm not sure that this was the exact best approach, but I went for a 1:1 translation for now.