Supported seed proposal: wdiff

How exactly did we forget to add this one? I use it all the time; it
makes debdiff work usefully. In general it's a handy addition to the set
of Unix text processing tools.

Added to https://www.ubuntulinux.org/wiki/SupportedSeedProposals.

-- 
Colin Watson                                    [cjwatson@xxxxxxxxxxxxx]