progressor(steps, props)

Add support for realitive sizes of progression steps, e.g.
```r
progress <- progressor(steps = 6L, props = c(1/3,3/3,2/3))
progress <- progressor(steps = 4L, props = c(1,3,2)) ## automatically rescaled
```
Alternatively,
```r
progress <- progressor(step_sizes = c(1L, 3L, 2L))
```
where `steps = length(step_sizes)` by default.

It would be neat if `props` can be modified along the way, e.g. if one of the steps end up finding a shortcut (because of input data) and only needing, say, 50% of its processing "time".