Add 'engine' field

[tzizi]

Allow specifying the game engine for original games. Most game remakes tend to remake the engine as well as the game, so this could be an interesting field. While I don't personally think anything else should come out of it right yet (such as having the games themselves specify the engine, as it's quite rare that a game uses an engine without modifying some part of it), I think it would be neat for users to be able to see what is the base engine each original game uses.

[cxong]

What if we use the existing framework field?

[tzizi]

I believe the framework field specifies what the remake has used, not what the original game used.