Added optional appDir option to don't depend on the cwd.

The working directory of my main program is 

    /something/main-prog

 but the plugin which uses nap is under 

    /something/some-plugin

and I don't want to change the cwd to the plugin folder. So I added a option for the parent folder of the plublic dir and the assets.

It would be really great if you could include the changes in your npm package.