Resolvable promise wrapper

This is a tiny promise wrapper which gives you promises which have explicit promise.resolve() / promise.reject() methods.

Eg:

``javascript
const resolvable = require('resolvable')

const myPromise = resolvable()

;(async () => {
doThingA()
await myPromise
doThingB()
})()

setTimeout(() => myPromise.resolve(), 1000)
``