[vim] Change the default layout to use popup window

The new default is

  { 'window' : { 'width': 0.9, 'height': 0.6, 'highlight': 'Normal' } }

The default highlight group for the border of the popup window is
'Comment', but 'Normal' seems to be a safer choice.

If you prefer the previous default, add this to your Vim configuration file:

  let g:fzf_layout = { 'down': '40%' }

(fzf will fall back to this if popup window is not supported)