Topic/stash stopwords

A number of dzil plugins are now poking into [%PodWeaver] for various things, including adding contributor stopwords... but really only *one* thing needs to add stopwords, and that's the code that actually weaves pod containing contributor names -- which is this module.  (I'll be updating those plugins too to remove the redundant code.)

(There isn't a clean API yet in Dist::Zilla::Role::Stash::Plugins for adding data - I'm working on that, and will update this code when it's complete.)

I also modified the code that extracts the data from the [%PodWeaver] stash to use the provided API.