Add flat-stop-condition function

Hello Community,

Have you thought about adding a function option that could act as a flat stop condition?

I'm having issues with flattening an ObjectId (from MongoDB). Currently Flat will split this in bsonType and id (with unknown chars).

If I could pass down a function to Flat that would receive the object to process and decide if it should be processed or continue it would be great no?

What do you think?

I'm available to create a PR if you feel that this is interesting.