Adding support for working with arbitrary assets to duo

Based on the discussion in MatthewMueller/file-deps#2, this allows `duo(1)` to accept files that are not known entry file types, and those files will be bundled up and copied like any other local asset.

```bash
duo images/*.png
# copies all png files in images/ to build/images/
```

**Note:** this does **not** work on directories, at least not yet.