Add black vim plugin

To help format python files.

Note some fiddling is required to install this correctly because

> This is the error resulting from incompatible versions of black.vim
> (the Vim plugin) and black.py (the Python library) we were previously
> discussing in #1293 -- you have the master version of black.vim, which
> however installs the latest release (i.e. code that's older than master)
> of black.py. Since the latest release of black.vim doesn't work with
> recent versions of Vim, your only option is to update black.py (so that
> both black.vim and black.py are the same bleeding edge -- master --
> version). As per the instructions you provided in the OP:

```
cd ~/.vim
black/bin/pip install --upgrade git+https://github.com/psf/black.git
```

From psf/black#1379 (comment)

home/.vimrc

@@ -18,6 +18,7 @@ call minpac#add('ervandew/supertab')
 call minpac#add('junegunn/fzf')
 call minpac#add('junegunn/fzf.vim')
 call minpac#add('ludovicchabant/vim-gutentags')
+call minpac#add('psf/black')
 call minpac#add('scrooloose/syntastic')
 call minpac#add('tpope/vim-commentary')
 call minpac#add('tpope/vim-dispatch')