Recursive bake detection

[legendecas]

With the issue of #19, I accidentally ran into an unexpected recursive bake invocation like:

foo:
  bake bar
bar:
  echo bar

And since shell would spawn a new process to run the bake, it is hard to get out of this indefinite loop. CTRL-C would not halt the situation. And pid resource would run out quickly until you luckily get a chance to force-quit all the bake process (like ps aux | grep bake | awk '{print $2}' | xargs kill -9).