Allow users to register hooks after an action

[kevinawoo]

Allow users to register their own hooks/callbacks via a script loader, or Templater, or User Plugins for Obsidian.

Users would then be able to do much more custom workflows when a task changes status.

An Example:

• When a task is marked as "Done", move file links with the task-* to archived/
• when a task is unDone, undo the move

Kind of relates to #582, this idea came about when I was thinking how I wanted to handle my Done tasks.