[Proposal] Add WaitUntil block

I would love to be able to support asynchronicity through a `WaitUntil [autocmd-events] [(comments)]` block, like this:

```
Execute (some asynchronous job):
MyAsynchronousCommand

WaitUntil BufNew,BufRead (new window pops up):
bufname('.') ==# '/tmp/foo' # Wait until a new buffer being created is /tmp/foo
# Other conditions here would be treated as AND conditions

Expect:
some content in /tmp/foo
```

If the `autocmd-events` are omitted, the WaitUntil would listen by default to a set of events (to determine) like BufNew, BufRead, BufWrite, CursorMove, ...

This would especially be helpful for asynchronous NeoVim plugins.

Thoughts?

(If you think that is a good idea but lack time, I can try to propose you a patch)

Florent