Add option to allow projects to be saved automatically

I found that I didn't like the prompt for saving projects, so I made it configurable, both on a global basis (using `project-persist-auto-save-global`) and on a project-basis (using `(pp/settings-set 'auto-save t)`).

The project setting will override the global setting in either capacity. For the project setting to be set to force the prompt, I had to check for the symbol `'prompt` rather than nil, to ensure that it was explicitly set.