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

@@ -6,6 +6,7 @@ call minpac#init()
 call minpac#add('k-takata/minpac', {'type': 'opt'})
 
 " add other plugins here.
+call minpac#add('psf/black')
 call minpac#add('kien/ctrlp.vim')
 call minpac#add('ElmCast/elm-vim')
 call minpac#add('JuliaEditorSupport/julia-vim')