Add proportional chop

[hughjonesd]

A proportional chop specifies proportions of range(x) to chop by. It is to chop_quantiles() as chop_evenly() is to chop_equally().

x <- 0:4
chop_prop(x, c(0.25, 0.75))
# same as
chop(x, min(x) + c(0.25, 0.75)*(max(x)-min(x)))