Addons can be placed in subfolders with the same name as the action,
in order to be able to e.g. clone git repos into the TODO_ACTIONS_DIR
rather than having to download addons and manage updates manually.
Closes#120
The todo_completion had the problem that it didn't consider the todo.cfg itself for the location of TODO_ACTIONS_DIR, it just tried the default location or a globally exported config value. With the injection of custom configuration now in place, we can actually delegate the listing to todo.sh itself.
The added built-in "listaddons" command is used for that; it may also be helpful for troubleshooting or to find out about available add-ons. (But the help / shorthelp commands offer more information.)
Additionally, completion is now more precise; only executable actions and no subdirs are listed now; this is also covered by the enhanced test.
The last bonus: The custom add-on actions are now only determined when there's actually completion of commands.
