Simplecov for decks

I would suggest "wagn_simplecov_filters" for this and "wagn_core_dev_simplecov_filters" in place of the ...for_gem one.

In general, I would suggest the mod-dev API should be treated as the "normal" API, and the core-dev is the exceptional one.  In fact, we might consider renaming "mod-dev" to just "dev".