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

src/core.go

@@ -188,6 +188,9 @@ func Run(opts *Options) (int, error) {
 			forward = false
 		case byBegin:
 			forward = true
+		case byPathname:
+			withPos = true
+			forward = false
 		}
 	}