feat: set defaults for each picker in telescope setup (#883)

This allows easier picker configuration for example:

```lua
require("telescope").setup {
  pickers = {
    buffers = {
      show_all_buffers = true,
      sort_lastused = true,
      theme = "dropdown",
      previewer = false,
      mappings = {
        i = {
          ["<c-q>"] = "smart_send_to_qflist",
        }
      }
    }
  }
}
```

This configuration will be applied when running `:Telescope buffers`

lua/telescope/init.lua

@@ -37,6 +37,7 @@ function telescope.setup(opts)
   end
 
   require('telescope.config').set_defaults(opts.defaults)
+  require('telescope.config').set_pickers(opts.pickers)
   _extensions.set_config(opts.extensions)
 end