An equivalant to TYPED_TEST from googletest

Hi google-benchmark team,

being accustomed to the googletest framework, I'm searching for an utility that matches TYPED_TEST.

I only found BENCHMARK_TEMPLATE, but this will pass all template arguments to just one benchmark. I'm searching for a way to do the same benchmark for a series of single types.

I have to repeat the same definition over and over again, just to benchmark the same thing for multiple types:
```
BENCHMARK_TEMPLATE(assign_char, gap);
BENCHMARK_TEMPLATE(assign_char, dna4);
BENCHMARK_TEMPLATE(assign_char, dna5);
BENCHMARK_TEMPLATE(assign_char, nucl16);
BENCHMARK_TEMPLATE(assign_char, rna4);
BENCHMARK_TEMPLATE(assign_char, rna5);
BENCHMARK_TEMPLATE(assign_char, char);
BENCHMARK_TEMPLATE(assign_char, union_composition);

BENCHMARK_TEMPLATE(to_char, gap);
BENCHMARK_TEMPLATE(to_char, dna4);
BENCHMARK_TEMPLATE(to_char, dna5);
BENCHMARK_TEMPLATE(to_char, nucl16);
BENCHMARK_TEMPLATE(to_char, rna4);
BENCHMARK_TEMPLATE(to_char, rna5);
BENCHMARK_TEMPLATE(to_char, char);
BENCHMARK_TEMPLATE(to_char, union_composition);
```

It would be nice to have a consistent behaviour between googletest and google/benchmark. Because, they complement each other quite nicely.

Thank you!