Added ability for 'after-skip' to be set to negative values.

Hi,
I needed to set the 'before-skip' and 'after-skip' spacing to negative values so that I could have really tight packed lists/tasks.
Negative values for 'before-skip' worked just fine, but not so much for 'after-skip'.
After some trial & error I came up with this little addition.
A good solution or will this lead to some problems down the road? (I'm not a TeX guru)