Adding options for default_dir, resource_root, and root_dir

[pseudomuto]

I'm working on an app which has a slightly different directory structure than the one expected by this library. In particular, I'd like the resources directory to be off the root of the project, and the preferences directory to be in ~/.config/<my-addin>.

This PR doesn't make those changes, which would be subjective and represent a breaking change. Instead I've made default_dir configurable via kwargs to the app constructor. I've also added a new option for resource_root, which defaults to command/resources to avoid any breaking changes for existing add ins.

@tapnair