Add a new submodule and then add to .vimrc
$ git submodule add https://github.com/<package repo>.git bundle/<package name>
To get Black to work, must have vim 7+ with Python3 support.
Install Python3 using Homebrew's default version. Install vim with Homebrew
$ brew install python vim
Add aliases in your bash profile to use the Homebrew installed version
$ alias python=/usr/local/bin/python3
$ alias pip=/usr/local/bin/pip3
Make sure you use the Homebrew vim not Mac vim
$ brew link vim