Generates the migration of Letterbox's table in the app's repo: the one that installs it, or, after an upgrade of Letterbox, the one that brings it to its latest version.
$ mix letterbox.gen.migration
$ mix letterbox.gen.migration --repo MyApp.Repo
It is mix phoenix_mountable.gen.migration Letterbox.Migrations: the
version the app is at is read from its migrations, and the new migration
pins the latest (see Letterbox.Migrations). Run it with mix ecto.migrate.
The options are those of mix phoenix_mountable.gen.migration, such as
--repo and --migrations-path.