Use the multiple property of rule operator instead of filter

[matthewma7]

I have similar needs as this https://github.com/mistic100/jQuery-QueryBuilder/issues/474

But as it is pointed by the author, changing the select element multiple attributes won't work (without special handling). https://github.com/mistic100/jQuery-QueryBuilder/issues/474#issuecomment-295874269.

But we have the multiple property at operator level. Could we use rule.operator.multiple instead of filter.multiple here? https://github.com/mistic100/jQuery-QueryBuilder/blob/dev/src/data.js#L378

[mistic100]

This is need a bigger investigation of the possible breaking changes this induces. But it makes sense :-)