I keep writing little lambdas to append things to list fields, so can you add two update operations to `tinydb.operations`, next to `increment`, `delete` and friends? `push(field, value)` should append `value` to the list stored in `field`, keeping the existing order, and `pull(field, value)` should remove every occurrence of `value` from that list, not just the first one. If a document doesn't have the field yet, `push` should create it as a one-element list, while `pull` should just leave that document alone without raising. If the field exists but isn't a list, `push` has to raise a `TypeError` instead of doing something weird with it. They should be used exactly like the existing operations, e.g. `db.update(push('tags', 'python'), User.name == 'john')`.