API suggestion for v4

It may be worthwhile in v4 to create an async interface, which has an option for either a callback or promise... For that matter, the existing zxcvbn could be placed into npm/cdnjs as, for example zxcvbn-data.js, and zxcvbn can be a thin wrapper that will async load in browser/browserify, or require in as a dependency in node.

This could also be a step towards supporting worker implementations where the actual work is done out of band in the browser and in node, sparing the main event loop.

I suggest the promise implementation expressly _not_ define a global `Promise`, and simply throw if it's called without a callback if `Promise` is undefined...

Alternatively, a wrapper that works this way could be created, that uses `zxcvbn` as-is. Something like `asdfgh` might be a good name for an async wrapper.

``` js
//default data source will be common cdn (cdnjs), with option to override
var zxcvbn = require('zxcvbn').setClientDataUrl('//foocdn/zxcvbn-data.js');
...
//error first callback style.
zxcvbn('password', function(err, result){
...
});
...
//ES6 Promise Implementation
zxcvbn('password')
.then(function(result){
...
})
.catch(function(err){
...
});
```

Start by reviewing the existing zxcvbn interface and its browser and Node loading paths. Before implementation, the project needs to choose between an integrated async API and a separate wrapper, then define callback, Promise, data-loading, and worker behavior as the completion criteria.

Written by the indexing model from the issue text.