Initiall support of preview scrollbar for neovim

[liuchengxu]

When the preview direction is LR for neovim, the scrollbar will be enabled automatically. If you'd like to customize the scrollbar or disable it,

" Change the fill char of scrollbar
let g:clap_preview = { 'scrollbar': { 'fill_char': '▌' } }

" Disable the scrollbar
let g:clap_preview = { 'scrollbar': { 'fill_char': '' } }