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)).
For safety this method should check that `index` is in the range `0..max 0 (length words - 1)`, otherwise it might lead to an exception later.