Fixes#106
While fixing the bug I've also extended the handler to print not just
where label starts but also where label ends.
Also implements #97, but for narratabble handler only.
This adds dependency on `unicode-width`, but it was already in the
dependency chain (`textwidth` depends on it). Although, previously it
was only for `"fancy"` feature, now it always required since narratable
handler is supported without `"fancy"`.