Add option to support non-error-first APIs

[sindresorhus]

Initially, I didn't want this as it's a bad practise, so I asked @SamVerschueren to make a module.

After using pify for a good while, I've needed it more often than I would have liked, with a few Node.js API's (fs.exists), events, Chrome extension API's, and browser API's.

Instead of pretty much duplicating most of the code here into rfpify, I think we should add an option for it here instead.

@SamVerschueren Thoughts?

[SamVerschueren]

I almost never have to use rfpify to be honest. But I'm not using as much obscure API'S as you do :p. So yes, it might be easier to add an option here.

[kevva]

I almost never have to use rfpify to be honest.

Me neither. Only if I need to catch some event when testing.

[notpushkin]

WebTorrent uses this sometimes:

client.add(magnetURI, function (torrent) {  // no error parameter
  // Got torrent metadata!
  console.log('Client is downloading:', torrent.infoHash)
})

[jdalton]

Just got bit by exists. I avoided the issue with:

pify((a, b) => fs.exists(a, (c) => b(null, c)))