vim plugin should support scoped configuration vars

[rpdelaney]

Problem statement

I use files in .vim/after/ftplugin to set filetype specific settings in vim. For example:

after/ftplugin/python.vim
---
autocmd BufWritePre <buffer> execute ':Black'
let g:black_linelength = 79

This pattern is useful because configuration variables irrelevant to the file type I'm handling are not set, and also I avoid executing every autocmd for every buffer I open.

However, the vim-black plugin does not read variables set in the local scope. Therefore, every time I open a buffer with filetype python, g:black_linelength is set. This could also give rise to conflicts in settings when files from different projects with different style conventions are open at the same time in different buffers.

Proposed solution

vim-black should provide support for configuration options at each appropriate scope level:

buffer-variable    b:     Local to the current buffer.
window-variable    w:     Local to the current window.
tabpage-variable   t:     Local to the current tab page.
global-variable    g:     Global.

Variables with the lowest scope should take precedence over others. For instance, if b:black_linelength is 120, but g:black_linelength is 79, Black should enforce line length 120.

Considered alternatives

N/A

If there is support for this I could try to put a PR together. (But be warned, my vimscript is even worse than my python ;)