cmd: add --progress-terminal-title to print ETA to terminal title

Adds a flag, --progress-terminal-title, that when used with --progress,
will print the string `ETA: %s` to the terminal title.

This also adds WriteTerminalTitle to lib/terminal

fs/config.go

@@ -107,6 +107,7 @@ type ConfigInfo struct {
 	StatsOneLineDateFormat string // If we want to customize the prefix
 	ErrorOnNoTransfer      bool   // Set appropriate exit code if no files transferred
 	Progress               bool
+	ProgressTerminalTitle  bool
 	Cookie                 bool
 	UseMmap                bool
 	CaCert                 string // Client Side CA