Promise support

[tjconcept]

1. What do you think of adding support for promises in this library, such that a sink returns a promise if no callback is supplied?
2. Would you accept a PR implementing it?

My rough idea for an implementation is along the lines of:

let p
if (!cb) {
  p = new Promise((resolve, reject) => {
    cb = err => err ? reject(err) : resolve()
  })
}

[existing code]

return p

[dominictarr]

not interested. if you want promises, make a wrapper module.