Support theme refences on fast scroller

[christophehenry]

Currently, the only way to set fast scroller color is by setting the fast_scroller_bar and fast_scroller_handle_idle colors in values/colors.xml. This prevents from using theme attributes. It would be nice that FastScroller is able to use theme attributes. Wether it is by setting fast_scroller_bar_color and fast_scroller_handle_idle_color attributes to FastScroller or allowing using ? notation in color definition.