Different color for progress marks

[andre-vm]

Hi! Thank you for this nice library!
It almost fits my needs, but I need different colors for the marks ahead and behind the thumb. I suggest adding a new attribute called tsb_tick_marks_progress_color (or similar, maybe this is too big) for the color of the marks behind the thumb and keep tsb_tick_marks_color for the color of the ones ahead of it. When not specified, tsb_tick_marks_progress_color may have the same value as tsb_tick_marks_color.

[andre-vm]

My bad! Just realized I can use a color selector to make this distinction. state_selected=true sets the color of marks behind the thumb.