[vim] Move `vimrc.after` into `darwin` tag

Otherwise `rcm` would not allow to override it for ubuntu.

We now have two perfectly distinguished files for each OS.
[rcrc] Merge EXCLUDE patterns and set default tag

- Multiple EXCLUDE patterns led to unintended behaviour on Ubuntu.
- Use `darwin` as a sane default to setup my Mac without extra tags

You need to provide the tag `ubuntu` for usage on Ubuntu though.
[README] Update content to reflect recent changes