Allow to pass external source for selection

[igorkamyshev]

Now it's impossible to create selection with some external dynamic source, but it could be useful for cases like search 👇

const foundTodos = createSelection(
  todos,
  {
    source: $searchString,
    fn: (todo, searchString): todo is ActiveTodo => todo.title.includes(searchString)
  },
)

[Minhir]

Looks like we need to have two fields source and clock to be consistent with sample. Also, there are two different cases:

1. You want to update selection on each $searchString update
2. You don't want to update selection on each $searchString update