Add support for a hook system

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Similar to Taskwarrior's hooks: https://taskwarrior.org/docs/hooks_guide.html

• post- and pre- prefixed lifetime hooks

Example use cases

• Sync ~/.tasklite folder with git on launch (pull from remote)
  and on exit (push to remote)
• Sync tasks between TaskLite and Habitica (as in taskwarrior-habitica-bridge)
• Refresh custom displays whenever a TaskLite command is run
• Show messages from a message server after each task command

More Ideas

• Support for hooks to be stored in the database in a scripting language which are then executed by TaskLite.
  They could e.g. be written in Lua (https://hackage.haskell.org/package/hslua) or Nix (https://github.com/haskell-nix/hnix). (Dhall is unfortunately not ergonomic enough).
  I always disliked the system for git hooks, in which they are not part of the project and everybody had to install them themselves.
  It could then even be extended with custom functions.
  This could lead to a collection of useful hook scripts that anyone can use, since they are executed by TaskLite and have no additional requirements for the system setup like installed interpreters and so on.

• Use Nix as the basis for a TaskLite plugin manager

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Tracking issue for @robwhitaker's request in #2 (comment)