added clear condition

As I anticipated, this was way easier to figure out than drawing the clear condition message. I hope to add another type of clear condition tomorrow where you have to reach a certain area in the level instead of just killing all the enemies. After that I'll add support for multiple levels and having different clear conditions in different levels which will definitely take a lot longer.

Overall I'm surprised by how quickly I'm getting stuff like this done. Earlier in the game's development it took me an entire month among other things to make the first enemy but now I can add a feature like this in under a week.