Feature Request: Add ability to set export name for joined exports.

[migoldfinger]

In Summary.cs line 92 the title and therefore the file name for exports are set to $"BenchmarkRun-joined-{DateTime.Now:yyyy-MM-dd-HH-mm-ss}"
If you run many sets of joined benchmarks it is hard do find the right one.

There should be an option to at least replace the DateTime part or the whole title with some custom string.