Allow to run migration without transaction

[smacker]

We have migration with CREATE INDEX CONCURRENTLY. It can't be done in transaction.
Will you be welcome for pull request similar to this commit? smacker@a56fc24

Migration example:

-- +goose DisableTransaction

-- +goose Up
-- SQL in section 'Up' is executed when this migration is applied

-- IF NOT EXISTS allows to run queries manually
-- CONCURRENTLY doesn't lock table, so it's safe to run on production without downtime
CREATE INDEX CONCURRENTLY
IF NOT EXISTS journey_last_miles_tracking_number_index
ON journey_last_miles (tracking_number);

-- +goose Down
-- SQL section 'Down' is executed when this migration is rolled back

DROP INDEX journey_last_miles_tracking_number_index;

[duffn]

This is already available with the NO TRANSACTION directive.

https://github.com/pressly/goose/pull/41/files

[smacker]

oh. I didn't follow updates. When I created my commit goose didn't have this feature. Sorry.