Transform "--" to "-{}-" in Tex files.

We want moves like "R--" and "D--" to be displayed as is, and not hyphened by Latex.

The clean way of doing this would have been to visit pandoc's AST but unfortunately the string "R--" is actually view as "R" then "-", then "-".
So I just used my old friend sed on the input.
It's still possible to add some more filter on the input file if needed.
This fixes #49.