[preview] Clean up preview scripts

- Use preview.sh instead of preview.rb by default
- Try bat syntax highlighter with the highest priority
- Remove "-v" option as the latest version of fzf sets up $LINES
- Allow users to customize the preview command via $FZF_PREVIEW_COMMAND
  - *EXPERIMENTAL / EVOLVING / UNDOCUMENTED*
  - Should be a command template with {} placeholder expression
    - e.g. "nl {}"

autoload/fzf/vim.vim

@@ -32,7 +32,7 @@ let s:is_win = has('win32') || has('win64')
 let s:layout_keys = ['window', 'up', 'down', 'left', 'right']
 let s:bin_dir = expand('<sfile>:h:h:h').'/bin/'
 let s:bin = {
-\ 'preview': s:bin_dir.(executable('ruby') ? 'preview.rb' : 'preview.sh'),
+\ 'preview': s:bin_dir.'preview.sh',
 \ 'tags':    s:bin_dir.'tags.pl' }
 let s:TYPE = {'dict': type({}), 'funcref': type(function('call')), 'string': type(''), 'list': type([])}
 if s:is_win