Queryable tags for entities

[Karl Ostmo]

I would like to categorize entities by custom tags, e.g.:

• In #1155, I would like to distinguish between entities that are "flowers" vs. "mushrooms"
• In #1628, I would like to distinguish between entities that are either "junk" or "useful", with perhaps subcategories of "edible" and "inedible".
• It may be useful to group together multiple entities as different "states" of the same thing, such as an "empty cask" and "water-filled cask", such as in #1599.

The implementation should be straightforward; just add a new field in the YAML description of an entity that accepts a list of strings (the "tags").
A new command hasTag : text -> text -> cmd bool that answers whether an entity has a given tag. This would be primarily intended for system robot use, though there is probably opportunity for creative uses as a scenario mechanic for players.