web: Allow downloading raw test results as CSV or JSON

[kamalmarhubi]

This would make it easier to get detailed data from folks without requiring them to download / run a command line tool. Useful if trying to get an idea of latency from a wide range of locations where you specifically want end-user latency.

An additional useful thing would be to allow controlling the number of iterations, and the download option via query parameters or fragment identifier. That way it's just one click for the person doing the test.

[kamalmarhubi]

Let me know if you'd accept a PR that does this. No promises, but looking at the code it seems this wouldn't require any big changes.