Allow for placing addons in subfolders.

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.

This comes in really handy if you also version control your .todo folder
in e.g. github, since this allows you to add the addon as a submodule
(assuming .todo/actions being your TODO_ACTIONS_DIR):

    cd ~/.todo
    git submodule add https://github.com/nthorne/todo.txt-cli-again-addon.git actions/again
    git submodule init
    git submodule update

This way, all you'll ever need to do in order to update all your addons is
    cd ~/.todo
    git submodule foreach git pull origin master