#601 Support TeX-style dashes!

[Adam Buchbinder]

Right now, we use either – or — to do
dashes. Just as ---- renders an <hr>, we should have --
render an en-dash and --- render an em-dash, as in TeX.
There's space in the parser for this markup.

If two dash variants would confuse people, having only
one would work as well, with just -- as valid dash
syntax. It'd end the timeless revert wars over
([[1805]]--[[1905]]) vs ([[1805]] &ndash; [[1905]]);
the former is readable in wikitext, the latter looks
nicer when rendered.

[Toby Bartels]

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Actually, "&ndash;" and "&mdash" ''don't'' looks nicer when
rendered on old browsers like Netscape 4; this is part of
the reason for the style wars. Even so, it's nicest (for
editors) when this sort of thing is handled by the software,
so I like this request.

Then my only worry is that the TeX-style input, while lovely
for input, is wrong for ASCII-style output. The ''correct''
style war is "&ndash;" vs "-" and "&mdash;" vs "--". OK, so
that is ''also'' subject to warring -- but it means that the
transition may be tricky.