edict: fix escape parsing and make exceptions configurable
In
312123ecffc7e3dd892c7df754c911b47b9cdfaa a bug was introduced where
newlines were no longer supported in string fields.
This patch supports them again, which requires exceptions be
reintroduced to fully support some buggy maps, but this time there's a
cvar for documentation and adjustment. The other notes and fixes in
https://github.com/DarkPlacesEngine/darkplaces/pull/185 still apply.
Signed-off-by: bones_was_here <bones_was_here@xonotic.au>