[feature request] Add tasks to a todo and add command for reindexing

[amanger-qual]

I'm loving nb and how it's letting me store information in a whole new way! I generally use glow to view the files because the nb show command show the front matter metadata.

I stared using nb to track programming tasks and new notebooks that I need to create; however i found that I can't add a task to a todo without editing the todo file myself. Could a command be introduced to add tasks to todos after the fact? It would also be great if todos indexed separately from notebooks, but maybe it works just fine for everyone else :)

I'd also love to be able to reindex all the nb items. When an item is deleted or moved, I currently need to open up the .index file and manually reorder the lines to get the index value assignment to be sequential and in the order I desire. If there was a command like nb reindex or nb index -r, that would make this a lot easier!

Again, love this project! I'd love to help contribute if I knew more about this kind of development, but my skill set isn't quite there. Hopefully one day soon!