Add async operators

[andres-kovalev]

Something similar to Promise.all(), but for streams:

import { create, asyncFilter, take } from 'pragmatic-streams';

take(2,
  asyncFilter(value => value > 1,
    create(
      i => Promise.resolve(i),
      5
    )
  )
);

or maybe something like:

...
take(2,
  async(
    filter(value => value > 1,
      create(
        i => Promise.resolve(i),
        5
      )
    )
  ) 
);