Support multiple BenchmarkMode

[Francesco Nigro]

Hi!

Given that Throughput and AverageTime benchmark modes are 2 views of the
same data, would be possible to get both on the benchmark results?

eg:

@BenchmarkMode(Mode.AverageTimeAndThroughput)
...

or

@BenchmarkMode({Mode.AverageTime, Mode.Throughput})


Benchmark                        (numItems)  Mode  Cnt      Score     Error
 Units
my_benchmark                             10  avgt   10          1 ±   0
  s/op
my_benchmark                             10  thrpt  10          1 ±   0
 ops/s


Thanks for this awesome tool!

Cheers,
Franz