--use should look for plugin in working directory

When doing something like

```bash
$ duo -r ./client index.css --use some-plugin > build.css
```
duo currently looks for the plugin in `./client/some-plugin` and `./client/node_modules/some-plugin`.
This patch makes it look in `nodes_modules` in the working directory as well.

Nice if you want to build from a Makefile in the root of the project.