Add support for multiple columns

[umairnadeem]

The following migration:

knex.schema.table('users', function (table) {
  table.string('first_name');
  table.string('last_name');
})

creates a query such as:

alter table "USERS" add "FIRST_NAME" varchar(255), **add** "LAST_NAME" varchar(255);

Whereas it should be:

alter table "USERS" add "FIRST_NAME" varchar(255), "LAST_NAME" varchar(255);

[umairnadeem]

Just realized we don't need this - it's already there for the mysql dialect.