Option to prioritize file name matches (#4192)

* 'pathname' is a new tiebreak option for prioritizing matches occurring
  in the file name of the path.

* `--scheme=path` will automatically set `--tiebreak=pathname,length`.

* fzf will automatically choose `path` scheme when the input is a TTY device,
  where fzf would start its built-in walker or run `$FZF_DEFAULT_COMMAND`
  which is usually a command for listing files.

Close #4191

plugin/fzf.vim

@@ -1081,7 +1081,7 @@ endfunction
 
 function! s:cmd(bang, ...) abort
   let args = copy(a:000)
-  let opts = { 'options': ['--multi'] }
+  let opts = { 'options': ['--multi', '--scheme', 'path'] }
   if len(args) && isdirectory(expand(args[-1]))
     let opts.dir = substitute(substitute(remove(args, -1), '\\\(["'']\)', '\1', 'g'), '[/\\]*$', '/', '')
     if s:is_win && !&shellslash