Allow the ability to specify a distPath

The distPath is used to tell Steal where it can find bundles. By default
bundles are located at `dist/bundles/` but with distPath you can specify
another location by doing:

```html
<script src="steal.js?app"
        data-dist-path="other/"
        data-env="production"></script>
```

Which will load the app at `other/bundles/app.js`.