support both per-project and per-user .ditz-plugin files

Ditz should search for a ~/.ditz-plugins, and load it, and then should search for a project .ditz-plugins, and then load that. This allows for both per-project and per-user plugins to be loaded, with the latter overriding the former.

Examples of per-user plugins include plugins that change or enhance the user interface, and examples of per-project plugins include plugins that integrate with a particular SCM.