Use knex defined stub if it exists

[dhensby]

Knex allows you define a migration stub in the knex config. At the moment this is ignored by knex-migrate which means that setting up a stub in knex migrations won't affect any newly generated migrations when using knex-migrate.

It would be good if it could use the migration stubs in the following order:

1. Stub passed by flag
2. Stub defined in knex config
3. Default stub