Use special comments to remove highlighting from selected areas

[ftilmann]

Introduce special comments to control latexdiff, in particular mark blocks where markup should be removed, e.g.

%DIFbegin nomarkup
some text and/or commands which should never be marked up
%DIFend nomarkup

For example, the material protected in this way would contain TeX commands, which cannot be process properly by latexdiff. Essentially, this would replace the role of the DIFnomarkup environment, which was always awkward because of the need to define a special environment in the source texts.