BulkQuery should support more complex `VALUES` clauses

[rowanseymour]

dbutil.BulkQuery currently only works when statement includes the exact text VALUES( but sometimes depending on formatting style it might be preferable to write it VALUES (.

Would be useful if it allowed casts as well, e.g. VALUES (:id::int) so that the column is created with type int and you don't have to use ::int elsewhere in the query.