Provide "meta" statistics

For different use cases, like PragTob/benchee_html#10 it'd be great to have statistics about statistics - what I'd call "meta statistics" - although there's probably some better real statistics name for this :)

What should be in there (that I know of so far):

* job size (how many jobs are in there)
* minimum of run times over all jobs
* maximum of run times over all jobs

This should be added as a new key to the benchmarking suite (`:meta_statistics`) - it could be added within the statistics module but might be better off as a separate `MetaStatistics` step that is computed after general statistics have been computed.

Start with the benchmarking suite and its statistics module, then compare whether a separate MetaStatistics step fits the existing computation flow. Done means the suite exposes a new :meta_statistics key containing job count, minimum runtime, and maximum runtime across all jobs.

Written by the indexing model from the issue text.