allow NONE color

[blockchainian]

this pr makes fzf gutter work for default background color NONE:

let g:fzf_colors = {
\   'gutter':  ['bg', 'Conceal']
\}

highlight Conceal ctermbg=NONE

• before

• after

[junegunn]

Thanks, but this is not compatible with the current model where you can specify multiple highlight groups.

let g:fzf_colors = {
\   'gutter':  ['bg', 'Conceal', 'Normal', 'Comment']
\}

highlight Normal ctermbg=233
highlight Conceal ctermbg=NONE

['bg', 'Conceal', 'Normal', 'Comment'] translates to "Use the background color of the first highlight group that has a background color", and that is 233 of Normal. But with your patch, it will choose -1 for Conceal, ignoring the rest of the list.