Continuation prompt not used for multiline hints

[thomcc]

You can see this in cargo run --example example after you get some multiline items in the history. It seems to be that the hint is just applied here directly, https://github.com/kkawakam/rustyline/blob/master/src/tty/unix.rs#L574-L580 (and basically the same for windows), instead of split up so that a continuation prompt can be applied.

Honestly it seems to me like it might be fine to just use spaces for the hint continuation prompt rather than further complicating prompt highlight code, otherwise it might be confusing... No strong feelings here though.