Make it possible to pass project path

For **ember-cli-app-version**,  @stefanpenner [suggested to read package.json from root instead of current working directory](https://github.com/taras/ember-cli-app-version/issues/3). This PR makes it possible to pass a path to the project. I will use this to pass relative project path when integrating to my library. 
@stefanpenner how about this?