Vertical preferences are ignored in Slider

[flightmansam]

Problem: Slider (Vertical) appears completely horizontal.

Fix:
slider.lua and theme.lua both set the slider for a special vertical case if "vertical" is in the opt table when calling the slider draw call. However a bug exists where the args are nested within in opts in a table parameter called ["id"]. To fix, you need to change slider.lua and theme.lua to access the vertical flag via opt.id.vertical.

[flightmansam]

#85