Entities can now be limited to attacking specific entity types

* Added a type property to entities
* Allowed entities to be limited to attacking only certain entity types, so enemies don't always attack each other.

I didn't commit the recompiled JS file from /dist so you would be less likely to run into merge conflicts. I can commit that file in the future if it makes things easier.

I've modified combat to take entity type into account here: https://github.com/stefanweck/dungeongeneration/pull/9/files#diff-b5fb76dd7831ab523605ab86d0304b21R61

Let me know what you think. I'm open to changes, as always.