Add label field to task definition

Label is an alternative name for task that replace it when printed in following context eg.:

- log: when given task is up to date and is skipped from execution
- log: when given task is NOT up to date (`--status` command)
- in `--summary` and `--list` commands output

help.go

@@ -21,7 +21,7 @@ func (e *Executor) PrintTasksHelp() {
 	// Format in tab-separated columns with a tab stop of 8.
 	w := tabwriter.NewWriter(e.Stdout, 0, 8, 0, '\t', 0)
 	for _, task := range tasks {
-		fmt.Fprintf(w, "* %s: \t%s\n", task.Task, task.Desc)
+		fmt.Fprintf(w, "* %s: \t%s\n", task.Name(), task.Desc)
 	}
 	w.Flush()
 }