stageRunners now show progress indicators

For example,

````R
sr <- stageRunner$new(new.env(), list(a = force, b = list(c = force, d = force, e = force)), remember = TRUE)
sr$run(1, to = '2/1')
print(sr)
````
yields
````
A caching stageRunner with 3 stages:
 + a
 * b
   + c
   * d
   - e
Context <environment: 0x10acb3c08>
````
which indicates `a` has been run, `b` is pending, `c` is finished, `d` is up next, and `e` has not been run yet.