Have an official location for system-wide user-installed plugins #2
Description
There should be an official location for system-wide user-installed plugins. See fullstaq-ruby/server-edition#24 for rationale.
In my opinion, /usr/lib/rbenv/plugins should be reserved for OS-package-manager-provided rbenv plugins. User-installed plugins should be looked up from elsewhere; I think /usr/local/lib/rbenv/plugins is preferred.