Migrations should be adapter-implemented

[davewasmer]

Right now migrations are tied to knex. We should uncouple them so that you write migrations in whatever syntax your ORM supports, and Denali just handles the running/state management of migrations.

Also, might be nice to add a bit of autogeneration, i.e. generate migration add-users would look for a User model, load the file, and handoff to ORM which could try to construct an appropriate migration from the class definition.

The downside of adapter-specific migrations is that addons requiring migrations now can't supply their own prebuilt, out of the box. This could be potentially partially mitigated by the autogeneration described above.