Add "addons" to pattern of non-plugin dir names

Ignores any directory called "addons" when scanning rtp for paths that look like plugin directories. This avoids the false positive of /var/lib/vim/addons/ showing up as a plugin on debian systems.

This means if you have a plugin called "addons" (shame on you!) maktaba will no longer correctly detect it. If we need to support that in the future, we could switch to a more clever heuristic like matching the full path.

Fixes #51.