Add rule for --.

Convert to &ndash; in html and - in txt. Leave as -- otherwise,
which works in TeX and LaTeX.
Improve '--' handling, update documentation.

Improve the -- handling: use postproc ' -- ' in HTML
and preproc ' -- ' in TXT. The surrounding spaces decrease
false positive hits, and preproc in TXT is used not to trigger
on underlines created by txt2tags itself.