enable db:rollback of batch

[jamemackson]

Per February 1, 2017 4:37 PM conversation in gitter, the db migrations in lux are not tracking a batch which would enable rolling back an entire batch at once enabling the ability to keep a production environment consistent which is critically important.

basically this would require, at minimum, adding a batch field to the migrations table and looking to this field for what migrations to rollback when calling lux db:rollback

[jamemackson]

@zacharygolba is this anywhere on your radar? Once I get my project a bit further along, I'll need this to be able to fit lux into our existing infrastructure / deployment process properly. I'm happy to work on a PR for this also if you were able to give me a nudge in the right direction also.