Future: Add support for migrations that just run existing SQL files

[Tobe Osakwe]

*This issue was automatically moved to: angel-dart/angel#225.

[Tobe Osakwe]

Example:

migrations: [
  MyMigration(),
  RawMigration(
    'lib/src/migrations/users_up.sql', 'lib/src/migrations/users_down.sql')
]