Method to scan for and register stylesheets

**Problem**
As web apps become more dynamic developers are now finding they have the need to load/unload stylesheets dynamically.  Currently, less does one initial scan for stylesheets on load.  This means less will not parse stylesheets added dynamically after page load, even when calling `less.refresh()` or `less.modifyVars()`.

**Proposed Solution**
Make the stylesheet loading code available as a method, returning a promise.  On resolve, you can `.then()` call a `less.refresh()` or `less.modifyVars()`:

```javascript
less.registerStylesheets().then(
    function () {
        less.modifyVars(record);
    }
);
```
So, my other PR includes the promise for refresh().  Should I merge it and make the changes you propose?