[FR] Register function using macro, write the loop part only

Hi. I was thinking about a way to register a test by writing as little as possible boilerplate and looking for something like:

```cpp
BENCHMARK_SIMPLE(TestName) {
// Benchmark subject
}
```

The downside that you have to write second `}` to at least end the function.
And as with `BENCHMARK_F`, you can't call methods afterwards.

```cpp
BENCHMARK_DEFINE_SIMPLE(TestName) {
// Benchmark subject
}

BENCHMARK_REGISTER_SIMPLE(TestName)->Setup(...)->Teardown(...);
```

Now we can call methods, but we have same thing as with `BENCHMARK_DEFINE_F`: you have to spell the name of the test twice. 100% no copypasta :smiling:

If we pass do as `BENCHMARK_CAPTURE` we will have an almost perfect solution.

```cpp
BENCHMARK_SIMPLE_CAPTURE([](benchmark::State& state){

})->Setup(...)->Teardown(...);
```

But we still have `state` argument written out. Dang it. We could remove it, but then we won't be able to get/set data from/to `state`.

I'll leave it as it for now, what do you think? Is it useful and is possible?