Added margin and padding support + mirrorring

Added:
-Margin (top, right, bottom, left): both rail and hrail
-Padding (top,right, bottom, left): both rail and hrail
-Option to mirror settings of rail to hrail when no specific margin or
padding is given to hrail (default:true)

Modified:
-Demo: document scroll is updated with newly added options to show and
test the modified code