Monsters flash when seen and when they see you

Hey sulai,

This is a small change to make monsters flash when you see them and also when they start hunting you, per suggestions on the forum.

I don't know how to add a branch to your repository, but you probably don't want to pull this into master.  If you make a `feature/monsterFlashes` branch or something, I can make another pull request if you like.

Incidentally, I saw an earlier comment indicating that you weren't currently active in the brogue community; if you don't want to deal with my pulls, I can make my own repository.  Don't feel obligated if it's not fun for you!