Make this plugin work with sbt subprojects

Make this plugin work with sbt subprojects, where package.json is in the subproject directory. In that case, we should explicitly pass the --prefix param to npm with the subproject's directory.

The directory structure would be like this:
```
root/
    build.sbt (root)
    project/
        plugins.sbt (containing sbt-js-engine)
    subproject/
        build.sbt (subproject)
        package.json (packages that subproject requires)
        node_modules/ (js modules that subproject can access)
```