Feat: Opt in vim buffers for previewers (#298)

Enable with:
require('telescope').setup {
  defaults = {
    file_previewer = previewers.vim_buffer_cat.new,
    grep_previewer = previewers.vim_buffer_vimgrep.new,
    qflist_previewer = previewers.vim_buffer_qflist.new,
  }
}

lua/telescope/config.lua

@@ -88,6 +88,10 @@ function config.set_defaults(defaults)
   set("file_sorter", sorters.get_fuzzy_file)
 
   set("file_ignore_patterns", nil)
+
+  set("file_previewer", function(...) return require('telescope.previewers').cat.new(...) end)
+  set("grep_previewer", function(...) return require('telescope.previewers').vimgrep.new(...) end)
+  set("qflist_previewer", function(...) return require('telescope.previewers').qflist.new(...) end)
 end
 
 function config.clear_defaults()