Add support to add new items on while running

[twolfson]

I have a long list of async tasks running that I only want to run 10 at a time (to avoid abusing CPU/RAM/IO). It would be nice to be able to add items while the process is running:

  ⠋ foo
  ⠋ bar

then

  ✔ foo
  ⠋ bar
  ⠋ baz

via some method like multispinner.addSpinner(key, val). I have a workaround setup here for anyone interested in the interim:

https://gist.github.com/twolfson/dd28b1c7aefb337a46419cc2a6972c98