Add support for promise

[ChenLi0830]

Make the url-exists support promises.

urlExists('https://www.google.com')
  .then(exists => {
    // do stuff
  })

[eric-hc]

I would use this package now: https://github.com/Richienb/url-exist

For anyone that came across this one and saw this PR and repo not being maintained.

[mma86-defy]

Use

import { promisify } from 'util';
import urlExists from 'url-exists';

const urlExistsAwaitable = promisify(urlExists);