Adjust the Global Executor thread count

I was wondering if there is any ability, or plans, to support setting the maximum number of threads for the global executor?

I see the `num_cpus` is used [here](https://github.com/async-rs/async-std/blob/8ea920c9f05f00fd8cc17c7e70ed5676e9a969ad/src/task/executor/pool.rs#L28) which is great 99% of the time, my use case, however, is for docker.

`num_cpus` can/will show more CPU's than is allocated for the container to use. I was hoping to be able to directly control this for situations like this.

Thanks you in advance for any information you can provide :)