2.0 Refactor theme and helper setup

Enable loading configuring helpers to load in `theme.json`.

For example, if you have a custom admin theme that requires a specialized helpers from a plugin, you can override the default helpers by specifying the following in `theme.json`:

```json
{
	"helpers" : {
		"prefixes" : {
			"" : {
				"Html": { "className": "MyPlugin.MyHtmlHelper"}
			},
			"admin" : {
				"Html": { "className": "Croogo.CroogoHtml" }
			}
		}
	}
}
```

You can have different helpers between prefixes and selectively loaded.