Handle up/down for multiple databases

This change adds the ability to run up/down for a database in a multi-db
environment.

If you have an app with a primary and animals database the following
tasks will be generated:

```
VERSION=123 rake db:migrate:up:primary
VERSION=123 rake db:migrate:up:primary

VERSION=123 rake db:migrate:down:primary
VERSION=123 rake db:migrate:up:animals
```

I didn't generate descriptions with them since we don't generate a
description for a single database application.

In addition to this change I've made it so if your application has
multiple databases Rails will raise if you try to run `up` or `down`
without a namespace. This is because we don't know which DB you want to
run `up` or `down` against unless the app tells us, so it's safer to
just block it and recommend using namespaced versions of up/down
respectively.

The output for the raise looks like:

```
You're using a multiple database application. To use `db:migrate:down`
you must run the namespaced task with a VERSION. Available tasks are
db:migrate:down:primary and db:migrate:down:animals.
```

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