ftintitle: add 'auto' option

Add auto option to move feat artist to title when importing.

Now that I look at this more closely, both the `try_write` and the `store` are unnecessary when this is invoked from the import hook. The import process takes care of that. (Sorry I didn't notice this change before!)

With that in mind, it might be better to keep the metadata management—writing and storing—in the command function. That way, `ft_in_title()` just does the core of the work (is easier to test, etc.).