Add migrations with multiple SQL files

[kraih]

The single file approach can get messy very quick if you have too many migrations. There should probably be another option where it uses a directory with multiple SQL files.

migrations/1/up.sql
migrations/1/down.sql
migrations/2/up.sql
migrations/3/up.sql
migrations/3/down.sql

Maybe like that. We'll have to see what works best for keeping many migration steps around.

[jhthorsen]

This is resolved by #72