Multi database migrations

As of #136 we can use multiple databases, but Avram can only migrate one. We should make it so we can migrate multiple

```crystal
PrimaryDatabase.configure do |settings|
settings.url = "some_url"
settings.migration_folder = "db/migrations/primary" # default is "db/migrations"
end

SpecialDatabase.configure do |settings|
settings.url = "some_url"
settings.migration_folder = "db/migrations/special"
end
```

Or maybe we set the repo in the migration file. Will require some experimenting

Extracted from #9

Start by reading issue #136 and the work extracted from #9, then locate Avram’s database configuration and migration entry points. Compare the proposed per-database migration_folder setup with associating a repository in each migration file. Done means multiple configured databases can be migrated while preserving the existing single-database behavior.

Written by the indexing model from the issue text.