Allow specifying data type other than app_dirs::AppDataType::UserConfig

[andybarron]

Add new versions of Preferences trait methods (e.g. save(..) -> save_type(..)) that let users specify the type of data to be saved/loaded (e.g. UserCache, UserData). Would expand support for non-config data such as cache and logs. (They can already be stored, but don't usually belong in the same place as config data.)