bench::press() should be more generic

[jimhester]

E.g. you should be able to use it with system_time() or really anything that returns a data.frame

[DavisVaughan]

Consider that bench::press() currently tries to return a bench_mark() object at the end. We could generalize it a little by only wrapping in a bench_mark class if all of the individual results were bench_marks. Then each press expression could return any arbitrary data frame.