`$skiptoken` support

[leviramsey]

I see that there's support in the parser for the $skiptoken option, but the SkipTokenOption gets dropped in QueryModelBuilder.

I'm prepared to add such support (e.g. by adding a SkipTokenOperation(op, token) QueryOperation), but before starting I figure it's prudent to ask if its absence is intentional (e.g. implementing something more important was made easier by not supporting it) or accidental (no one needed it).