Update tokio example

[rgreinho]

Updates the tokio with an example where a main progress bar monitors child
progress bars started asynchronously.

This examples also remove the blocking call in the async function, an uses the
#[tokio::main] macro instead of initializing the runtime manually.

[rgreinho]

This PR is a follow up of #408.

[djc]

I'm sorry, I should maybe have looked at this code more closely before suggesting you merge this into an existing example. I don't think this change makes sense as it doesn't seem to cover particularly new ground -- to me, the integration of a MultiProgress bar with async/await code seems fairly straightforward.

(This PR also makes other undesirable changes, like pulling a futures dependency into the library dependencies even though it's only used for an example -- that should at most be a dev-dependency, and it looks like we only need futures-util here.)

I appreciate the effort to contribute here, but I think this will not help -- people are more likely to need a single ProgressBar example in async code, and if they want to see how MultiProgress works they can look at other examples that show how that works.