Add functions that work with Promises

Nobody has claimed this yet.

In may scenarios things are simplified with Promises. It'd be handy to have some built into Dust e.g. (for illustration)

```javascript
Object.assign(dust, {
asyncRenderSource(str='', view={}) {
return new Promise((res, rej) => {
const template = dust.loadSource(dust.compile(str, ''))
try {
dust.render(template, view, (err, out) => err ? rej(err) : res(out))
} catch(e) { rej(e) } // see #107
})
}
})
```
Noting #107 re. syntax errors.

Cheers . :beers:

The issue names no files or tests; start by reading Dust's compile, loadSource, and render entry points, then review the syntax-error context in #107. Done means the agreed Promise-based Dust functions are defined and covered by tests for successful rendering and error handling, though the issue does not specify the full API scope.

Written by the indexing model from the issue text.