Fix getNumberSlidesShowing

getNumberSlidesShowing returned incorrect number of slides, because it ignored margins and paddings

sorry for such a dirty commit, it seems your ide added whitespaces on line break.
all changes inside getNumberSlidesShowing declaration.
https://github.com/mozzi/bxslider-4/compare/fix-getNumberSlidesShowing?expand=1#L1R411