[FR] Add possibility to start iteration in timing-off state.

A common pattern I am using in my benchmarking code is:
```
for (auto _ : state) {
state.PauseTiming();
/* Generate some data for this iteration, or cleanup from the previous */
state.ResumeTiming();

/* Do some work */
}
```

This seems a little unpleasant because timing is starting when entering the loop body and immediately being paused.
What I would like instead is something like:
```
for (auto _ : state.with_paused_timing()) {
/* Generate some data for this iteration, or cleanup from the previous */
state.ResumeTiming();

/* Do some work */
}
```