Support for using alter table

[billybonks]

ALTER TABLE the_table ALTER COLUMN col_name TYPE integer USING (col_name::integer)

I tried something like this

exports.up = function(pgm) {
  pgm.alterColumn('pull_request_reviews', 'repo_member_id', {
    type: 'BIGINT',
    using: '(pull_request_reviews::bigint)'
  });
};