Process: system() should use jobstart() on neovim

[josephholsten]

#469 causes system() to no longer support background processes on neovim. It looks like the replacement is jobstart().

I'd love to implement myself, but my vimscript is terrible, so I'd appreciate a little help!

[lambdalisue]

https://github.com/vim-jp/vital.vim/pull/513 would help