Pull stream drain

A modern version of the
pull-stream drain sink which returns a
promise rather than calling a callback.

``js
const {pull, values} = require('pull-stream')
const drain = require('psp-drain')

pull(values([1, 2, 3]), drain(console.log)).then(() => console.log('Done'))
`

`
drain([op])
`

Abort

`js
const {pull, values} = require('pull-stream')
const drain = require('psp-drain')

const sink = drain()
sink.abort()
pull(values([1, 2, 3]), sink).then(() => console.log('Done'))

const sink = drain()
sink.abort(new Error())
pull(values([1, 2, 3]), sink).catch(console.error)
`

`
abort([err])
``