Experimentally use wildmenu

Inspired by #265, set a `wildmenu` and `wildmode` that reduces visual
churn when using filename completion. See the arguments in favor on that
pull request.

This introduces an `experimental` tag. The idea is that people who want
to try the newest thing can "subscribe" to that tag, and once they pass
muster we can move them elsewhere.

For this to work, the `.vimrc` must load in arbitrary files; I picked
the `.vim/configs` directory for this.