Add dynamic slider range and step update

Useful to have such thing when slider ranges is not known on initialization or they are changed during runtime. Most of sliders providers have such feature. Would be great to have it in this wrapper.

I am not sure if it is the right way to do it but angular scope.$watch()'es fits here. On the other hand attrs.$observe() might be more performance friendly way. Also default default range min/max values should be set. Using those 0/100 respectively.