* todos.el: Allow prompting for first item on adding a new

* todos.el: Allow prompting for first item on adding a new
category.  Fix handling of default todo file on adding first file.
(todos-add-item-if-new-category): New defcustom.
(todos-show, todos-jump-to-category, todos-add-category): Use it.
(todos-short-file-name): Return nil if file name is not a string.
(todos-add-file): Recalculate todos-files after adding a file.
(todos-show): If first file was just added, make it the default.