Add increment option to sliders

[eprovst]

Fairly often I have a variable I want to keep an integer (eg. for Chebyshev polynomials y = cos(N acos(x)) we only use integer N). In similar software there is usually an option to set the increment of a slider, to stick to the integers one would then select an increment equal to 1, as far as I can tell this is currently not possible in Plots.