Support greedy / immediate result reporting

Instead of accumulating all results and then passing them off to a reporter like we do now, it is sometimes beneficial to report results as soon as they are available (for example in a console).

This has some implications:

* First of all, the runner requires a slot for a `BenchmarkReporter` instance in its run method.
* Secondly, for anything other than a console reporter, this could incur higher overhead than just a batched reporting, e.g. over a connection (HTTP, streaming, file writing). Maybe a "greedy" reporting could be a "batched reporting with batch size = 1" scenario.
* Thirdly, for anything table-based, any non-static ("unbounded" in Google Dataflow lingo) data stream requires certain parameters up-front for correct rendering (e.g. max column width). These either have to be inferred or set upfront by some heuristic.

Look at the runner's run method and the BenchmarkReporter class to understand the current batched reporting flow. Identify where results are accumulated and passed to the reporter. Consider how to modify the reporter interface to accept results immediately, and handle implications for different reporter types like console, file, or streaming. Check for existing tests around reporting to ensure changes don't break existing functionality.

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