Async example

[nu11ptr]

Added an asynchronous echo example to only show completions after a given delay in time. This is useful if, for example, the completion source has latency before completion data can be filled. By doing this, typing remains fluid and not delayed by the suggestions, and operations like copy/paste remain fast. Still, if the user stops typing for a moment and needs completion assistance, they will then pop up as expected.

[marco-m]

@c-bata would you have time to review this PR? thanks!