Detects margins around the rails

Hi and thanks a lot for this perfect scrollbar!

I had the need to add margins at the top and bottom of the Y rail, so I made the changes to enable this functionality after unsuccessfully trying to find a workaround with borders.

The only thing needed is to add `margin-*` CSS properties to the `ps-scrollbar-*-rail`.

I'd say it could be a good thing to have that functionality in the next release to make this plugin even more perfect, but that's for you to say.