dustc: provide an output-dir setting

When --split is turned on, templates are compiled in place. It would be useful to create a separate identical directory structure.

For example, if templates are located at:

```
templates/foo.dust
templates/include/bar.dust
```

It would be useful to compile them to

```
compiled/foo.js
compiled/include/bar.js
```

Start by locating the dustc entry point and the handling for --split. Trace how template paths become compiled files, then verify that an output directory preserves the example's directory structure while leaving source templates unchanged.

Written by the indexing model from the issue text.