Fix LeaderF bug

It will add files to buffer list even if we have not confirmed the
selection. This behavior is really annoying.
See also https://github.com/Yggdroot/LeaderF/commit/27bd938a422fc6fcfc453fa88fe2ad453a012074.

core/plugins.vim

@@ -109,6 +109,20 @@ let g:Lf_PopupColorscheme = 'gruvbox_material'
 " items.
 let g:Lf_CommandMap = {'<C-J>': ['<C-N>'], '<C-K>': ['<C-P>']}
 
+" do not preview results, it will add the file to buffer list
+let g:Lf_PreviewResult = {
+      \ 'File': 0,
+      \ 'Buffer': 0,
+      \ 'Mru': 0,
+      \ 'Tag': 0,
+      \ 'BufTag': 1,
+      \ 'Function': 1,
+      \ 'Line': 0,
+      \ 'Colorscheme': 0,
+      \ 'Rg': 0,
+      \ 'Gtags': 0
+      \}
+
 """"""""""""""""""""""""""""open-browser.vim settings"""""""""""""""""""
 if g:is_win || g:is_mac
   " Disable netrw's gx mapping.