Fix and enhance en/em dash translation

Depending on the back-end being used, `--` gets currently translated either to an em dash (HTML, for instance) or an en dash (LaTeX). To allow convenient usage of both types of dashes and resolve this inconsistency, translate (in sync with LaTeX) `--` to en dash and `---` to em dash.

Well, this will "break" existing documents, of course—the AsciiDoc user guide, for instance.