add --list (or -l) flag to print existing tasks

If an inexixtent task is given, the help also prints as before

Also fixing README documentation

Closes #51

help.go

@@ -6,7 +6,8 @@ import (
 	"text/tabwriter"
 )
 
-func (e *Executor) printExistingTasksHelp() {
+// PrintTasksHelp prints help os tasks that have a description
+func (e *Executor) PrintTasksHelp() {
 	tasks := e.tasksWithDesc()
 	if len(tasks) == 0 {
 		return
@@ -16,7 +17,7 @@ func (e *Executor) printExistingTasksHelp() {
 	// 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.Fprintln(w, fmt.Sprintf("- %s:\t%s", task, e.Tasks[task].Desc))
+		fmt.Fprintln(w, fmt.Sprintf("* %s:\t%s", task, e.Tasks[task].Desc))
 	}
 	w.Flush()
 }