LTerm_read_line: Allow to define the index with set_completion.

This allows to set the index of completion when setting the completion. It allows to "focus" a specific completion directly (which I need for [this](https://github.com/OCamlPro/ocp-index/pull/43)).
Sorry, just saw your answer. I think it's best to raise an error, except if `words` is empty. `Invalid_argument` seems fine.