Fix https://github.com/junegunn/fzf.vim/issues/1131

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)

Fix https://github.com/junegunn/fzf.vim/issues/1116

e.g.

  if exists('$TMUX')
    let g:fzf_layout = { 'tmux': '-p90%,60%' }
  else
    let g:fzf_layout = { 'window': { 'width': 0.9, 'height': 0.6 } }
  endif

e.g.

  let g:fzf_layout = { 'window': { 'width': 0.4, 'height': 1, 'xoffset': 0, 'border': 'right' } }
  let g:fzf_layout = { 'window': { 'width': 0.4, 'height': 1, 'xoffset': 1, 'border': 'left' } }
  let g:fzf_layout = { 'window': { 'width': 1, 'height': 0.5, 'yoffset': 1, 'border': 'top' } }
  let g:fzf_layout = { 'window': { 'width': 1, 'height': 0.5, 'yoffset': 0, 'border': 'bottom' } }

Close https://github.com/junegunn/fzf.vim/issues/942

e.g.
  let g:fzf_layout = { 'window': { 'width': 0.9, 'height': 0.6 } }

Based on the code from https://github.com/junegunn/fzf.vim/issues/821#issuecomment-581273191
by @lacygoill.

Update copyright year to 2020 and change to multi-year format.

The section is no longer relevant since (G)Vim 8 or above has builtin
terminal emulator.

Close #1055

https://github.com/junegunn/fzf.vim/issues/185

Close #893